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/templates/login.html

@@ -0,0 +1,14 @@
+{% extends "layout.html" %}
+{% block body %}
+  <h2>Login</h2>
+  {% if error %}<p class="error"><strong>Error:</strong> {{ error }}{% endif %}
+  <form action="{{ url_for('login') }}" method="post">
+    <dl>
+      <dt>Username:
+      <dd><input type="text" name="username">
+      <dt>Password:
+      <dd><input type="password" name="password">
+      <dd><input type="submit" value="Login">
+    </dl>
+  </form>
+{% endblock %}