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/tests/context.py

@@ -0,0 +1,6 @@
+import sys, os
+
+basedir = os.path.dirname(os.path.abspath(__file__))
+sys.path.insert(0, basedir + '/../')
+
+from flaskr import flaskr

examples/flaskr/tests/test_flaskr.py

@@ -0,0 +1,76 @@
+# -*- coding: utf-8 -*-
+"""
+    Flaskr Tests
+    ~~~~~~~~~~~~
+
+    Tests the Flaskr application.
+
+    :copyright: (c) 2015 by Armin Ronacher.
+    :license: BSD, see LICENSE for more details.
+"""
+
+import pytest
+import os
+import tempfile
+
+from context import flaskr
+
+@pytest.fixture
+def client(request):
+    db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
+    flaskr.app.config['TESTING'] = True
+    client = flaskr.app.test_client()
+    with flaskr.app.app_context():
+        flaskr.init_db()
+
+    def teardown():
+        os.close(db_fd)
+        os.unlink(flaskr.app.config['DATABASE'])
+    request.addfinalizer(teardown)
+
+    return client
+
+
+def login(client, username, password):
+    return client.post('/login', data=dict(
+        username=username,
+        password=password
+    ), follow_redirects=True)
+
+
+def logout(client):
+    return client.get('/logout', follow_redirects=True)
+
+
+def test_empty_db(client):
+    """Start with a blank database."""
+    rv = client.get('/')
+    assert b'No entries here so far' in rv.data
+
+
+def test_login_logout(client):
+    """Make sure login and logout works"""
+    rv = login(client, flaskr.app.config['USERNAME'],
+               flaskr.app.config['PASSWORD'])
+    assert b'You were logged in' in rv.data
+    rv = logout(client)
+    assert b'You were logged out' in rv.data
+    rv = login(client, flaskr.app.config['USERNAME'] + 'x',
+               flaskr.app.config['PASSWORD'])
+    assert b'Invalid username' in rv.data
+    rv = login(client, flaskr.app.config['USERNAME'],
+               flaskr.app.config['PASSWORD'] + 'x')
+    assert b'Invalid password' in rv.data
+
+
+def test_messages(client):
+    """Test that messages work"""
+    login(client, flaskr.app.config['USERNAME'],
+          flaskr.app.config['PASSWORD'])
+    rv = client.post('/add', data=dict(
+        title='<Hello>',
+        text='<strong>HTML</strong> allowed here'
+    ), follow_redirects=True)
+    assert b'No entries here so far' not in rv.data
+    assert b'&lt;Hello&gt;' in rv.data
+    assert b'<strong>HTML</strong> allowed here' in rv.data