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/static/style.css

@@ -0,0 +1,18 @@
+body            { font-family: sans-serif; background: #eee; }
+a, h1, h2       { color: #377BA8; }
+h1, h2          { font-family: 'Georgia', serif; margin: 0; }
+h1              { border-bottom: 2px solid #eee; }
+h2              { font-size: 1.2em; }
+
+.page           { margin: 2em auto; width: 35em; border: 5px solid #ccc;
+                  padding: 0.8em; background: white; }
+.entries        { list-style: none; margin: 0; padding: 0; }
+.entries li     { margin: 0.8em 1.2em; }
+.entries li h2  { margin-left: -1em; }
+.add-entry      { font-size: 0.9em; border-bottom: 1px solid #ccc; }
+.add-entry dl   { font-weight: bold; }
+.metanav        { text-align: right; font-size: 0.8em; padding: 0.3em;
+                  margin-bottom: 1em; background: #fafafa; }
+.flash          { background: #CEE5F5; padding: 0.5em;
+                  border: 1px solid #AACBE2; }
+.error          { background: #F0D6D6; padding: 0.5em; }