Address #1902: Converts example/flaskr to have a setup.py (#1945)

* Converts example/flaskr to have a setup.py

Makes the flaskr app easier to run, ex. workflow:
- pip install --editable .
- export FLASK_APP=flaskr.flaskr
- flask initdb
- flask run

Testing is also easier now:
- python setup.py test

* Fixed an import error in flaskr/tests

- the statement `import flaskr` caused errors in python3
- `from . import flaskr` fixes the issue in 2.7.11 and 3.5.1

* Better project structure and updates the docs

- Re-factors *flaskr*'s project structure a bit
- Updates docs to make sense with the new structure
  - Adds a new step about installing Flask apps with setuptools
  - Switches first-person style writing to second-person (reads better IMO)
  - Adds segments in *testing.rst* for running tests with setuptools

* Remove __init__.py from tests

- py.test recommends not using __init__.py

* Fix testing import errors

docs/tutorial/schema.rst

@@ -3,10 +3,10 @@
 Step 1: Database Schema
 =======================
 
-First, we want to create the database schema. Only a single table is needed
-for this application and we only want to support SQLite, so creating the
-database schema is quite easy. Just put the following contents into a file
-named `schema.sql` in the just created `flaskr` folder:
+In this step, you will create the database schema.  Only a single table is
+needed for this application and it will only support SQLite.  All you need to do
+is put the following contents into a file named :file:`schema.sql` in the
+:file:`flaskr/flaskr` folder:
 
 .. sourcecode:: sql
 
@@ -17,7 +17,7 @@ named `schema.sql` in the just created `flaskr` folder:
       'text' text not null
     );
 
-This schema consists of a single table called ``entries``. Each row in
+This schema consists of a single table called ``entries``.  Each row in
 this table has an ``id``, a ``title``, and a ``text``.  The ``id`` is an
 automatically incrementing integer and a primary key, the other two are
 strings that must not be null.