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

examples/flaskr/flaskr.py

@@ -1,110 +0,0 @@
-# -*- coding: utf-8 -*-
-"""
-    Flaskr
-    ~~~~~~
-
-    A microblog example application written as Flask tutorial with
-    Flask and sqlite3.
-
-    :copyright: (c) 2015 by Armin Ronacher.
-    :license: BSD, see LICENSE for more details.
-"""
-
-import os
-from sqlite3 import dbapi2 as sqlite3
-from flask import Flask, request, session, g, redirect, url_for, abort, \
-     render_template, flash
-
-
-# create our little application :)
-app = Flask(__name__)
-
-# Load default config and override config from an environment variable
-app.config.update(dict(
-    DATABASE=os.path.join(app.root_path, 'flaskr.db'),
-    DEBUG=True,
-    SECRET_KEY='development key',
-    USERNAME='admin',
-    PASSWORD='default'
-))
-app.config.from_envvar('FLASKR_SETTINGS', silent=True)
-
-
-def connect_db():
-    """Connects to the specific database."""
-    rv = sqlite3.connect(app.config['DATABASE'])
-    rv.row_factory = sqlite3.Row
-    return rv
-
-
-def init_db():
-    """Initializes the database."""
-    db = get_db()
-    with app.open_resource('schema.sql', mode='r') as f:
-        db.cursor().executescript(f.read())
-    db.commit()
-
-
-@app.cli.command('initdb')
-def initdb_command():
-    """Creates the database tables."""
-    init_db()
-    print('Initialized the database.')
-
-
-def get_db():
-    """Opens a new database connection if there is none yet for the
-    current application context.
-    """
-    if not hasattr(g, 'sqlite_db'):
-        g.sqlite_db = connect_db()
-    return g.sqlite_db
-
-
-@app.teardown_appcontext
-def close_db(error):
-    """Closes the database again at the end of the request."""
-    if hasattr(g, 'sqlite_db'):
-        g.sqlite_db.close()
-
-
-@app.route('/')
-def show_entries():
-    db = get_db()
-    cur = db.execute('select title, text from entries order by id desc')
-    entries = cur.fetchall()
-    return render_template('show_entries.html', entries=entries)
-
-
-@app.route('/add', methods=['POST'])
-def add_entry():
-    if not session.get('logged_in'):
-        abort(401)
-    db = get_db()
-    db.execute('insert into entries (title, text) values (?, ?)',
-               [request.form['title'], request.form['text']])
-    db.commit()
-    flash('New entry was successfully posted')
-    return redirect(url_for('show_entries'))
-
-
-@app.route('/login', methods=['GET', 'POST'])
-def login():
-    error = None
-    if request.method == 'POST':
-        if request.form['username'] != app.config['USERNAME']:
-            error = 'Invalid username'
-        elif request.form['password'] != app.config['PASSWORD']:
-            error = 'Invalid password'
-        else:
-            session['logged_in'] = True
-            flash('You were logged in')
-            return redirect(url_for('show_entries'))
-    return render_template('login.html', error=error)
-
-
-@app.route('/logout')
-def logout():
-    session.pop('logged_in', None)
-    flash('You were logged out')
-    return redirect(url_for('show_entries'))