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/README

@@ -13,15 +13,19 @@
          export an FLASKR_SETTINGS environment variable
          pointing to a configuration file.
 
-      2. Instruct flask to use the right application
+      2. install the app from the root of the project directory
 
-         export FLASK_APP=flaskr
+         pip install --editable .
 
-      3. initialize the database with this command:
+      3. Instruct flask to use the right application
+
+         export FLASK_APP=flaskr.flaskr
+
+      4. initialize the database with this command:
 
          flask initdb
 
-      4. now you can run flaskr:
+      5. now you can run flaskr:
 
          flask run
 
@@ -30,5 +34,5 @@
 
     ~ Is it tested?
 
-      You betcha.  Run the `test_flaskr.py` file to see
+      You betcha.  Run `python setup.py test` to see
       the tests pass.