Replace MongoKit by MongoEngine on docs

Docs chapter "Patterns for Flask" has a section dedicated to MongoKit,
but MongoKit lib seems unmaintained and is not compatible with recent
versions of PyMongo.

This commit replaces MongoKit guide with a new one using MongoEngine.

Closes #2907

docs/patterns/mongoengine.rst

@@ -0,0 +1,153 @@
+.. mongoengine-pattern:
+
+MongoEngine in Flask
+====================
+
+Using a document database rather than a full DBMS gets more common these days.
+This pattern shows how to use MongoEngine, a document mapper library, to
+integrate with MongoDB.
+
+This pattern requires a running MongoDB server, MongoEngine_ and Flask-MongoEngine_
+libraries installed::
+
+    pip install flask-mongoengine
+
+.. _MongoEngine: http://mongoengine.org
+.. _Flask-MongoEngine: http://docs.mongoengine.org/projects/flask-mongoengine/en/latest/>`_
+
+Configuration
+-------------
+
+Basic setup can be done by defining ``MONGODB_SETTINGS`` on App config and then
+creating a ``MongoEngine`` instance::
+
+    from flask import Flask
+    from flask_mongoengine import MongoEngine
+
+    app = Flask(__name__)
+    app.config['MONGODB_SETTINGS'] = {
+        'host': "mongodb://localhost:27017/mydb"
+    }
+    db = MongoEngine(app)
+
+
+Mapping Documents
+-----------------
+
+To declare models that will represent your Mongo documents, just create a class that
+inherits from ``Document`` and declare each of the fields::
+
+    from mongoengine import *
+
+
+    class Movie(Document):
+
+        title = StringField(required=True)
+        year = IntField()
+        rated = StringField()
+        director = StringField()
+        actors = ListField()
+
+If the model has embedded documents, use ``EmbeddedDocument`` to defined the fields of
+the embedded document and ``EmbeddedDocumentField`` to declare it on the parent document::
+
+    class Imdb(EmbeddedDocument):
+
+        imdb_id = StringField()
+        rating = DecimalField()
+        votes = IntField()
+
+
+    class Movie(Document):
+
+        ...
+        imdb = EmbeddedDocumentField(Imdb)
+
+
+Creating Data
+-------------
+
+Just create the objects and call ``save()``::
+
+    bttf = Movie(title="Back To The Future", year=1985)
+    bttf.actors = [
+        "Michael J. Fox",
+        "Christopher Lloyd"
+    ]
+    bttf.imdb = Imdb(imdb_id="tt0088763", rating=8.5)
+    bttf.save()
+
+
+Queries
+-------
+
+Use the class ``objects`` attribute to make queries::
+
+    bttf = Movies.objects(title="Back To The Future").get()  # Throw error if not unique
+
+``objects`` is an iterable. Query operators may be user by concatenating it with the document
+key using a double-underscore::
+
+    some_theron_movie = Movie.objects(actors__in=["Charlize Theron"]).first()
+
+    for recents in Movie.objects(year__gte=2017):
+        print(recents.title)
+
+Available operators are as follows:
+
+* ``ne`` -- not equal to
+* ``lt`` -- less than
+* ``lte`` -- less than or equal to
+* ``gt`` -- greater than
+* ``gte`` -- greater than or equal to
+* ``not`` -- negate a standard check, may be used before other operators (e.g.
+  ``Q(age__not__mod=5)``)
+* ``in`` -- value is in list (a list of values should be provided)
+* ``nin`` -- value is not in list (a list of values should be provided)
+* ``mod`` -- ``value % x == y``, where ``x`` and ``y`` are two provided values
+* ``all`` -- every item in list of values provided is in array
+* ``size`` -- the size of the array is
+* ``exists`` -- value for field exists
+
+String queries
+::::::::::::::
+
+The following operators are available as shortcuts to querying with regular
+expressions:
+
+* ``exact`` -- string field exactly matches value
+* ``iexact`` -- string field exactly matches value (case insensitive)
+* ``contains`` -- string field contains value
+* ``icontains`` -- string field contains value (case insensitive)
+* ``startswith`` -- string field starts with value
+* ``istartswith`` -- string field starts with value (case insensitive)
+* ``endswith`` -- string field ends with value
+* ``iendswith`` -- string field ends with value (case insensitive)
+* ``match``  -- performs an $elemMatch so you can match an entire document within an array
+
+Some Tips
+---------
+
+* Attributes can be set as ``unique``
+* ``MongoEngine`` creates the ``_id`` attribute automatically to acess ``ObjectIds``
+* You can add choices to string fields: ``StringField(choices=['Apple', 'Banana'])``
+* If you don't want your class name to be the same name as the collection, you can define
+  a ``meta`` class member and use the ``collection`` parameter::
+
+    class Movie(Document):
+
+        meta ={'collection': 'movie_documents'}
+
+Accessing PyMongo MongoClient
+-----------------------------
+
+If, for some reason, you want to access PyMongo instance, use ``get_connection`` function::
+
+    from mongoengine.connection import get_connection
+
+    conn = get_connection()
+    collection = conn.mydb.movie
+    collection({'title': u'Days of Thunder'})
+
+For more information about MongoEngine, head over to the
+`website <http://docs.mongoengine.org/>`_.