Better workflow for flaskr and other basic apps (pallets#2000)

- adds `from flaskr import app` to top-level in flaskr module
- effect is that `export FLASK_APP=flaskr` works over the more verbose
  `export FLASK_APP=flaskr.flask`
- see the readme for how to run
- all tests are passing with `py.test` or `python setup.py test` (in
  venv)

Author: wgwz

examples/flaskr/README

@@ -19,7 +19,7 @@
 
       3. Instruct flask to use the right application
 
-         export FLASK_APP=flaskr.flaskr
+         export FLASK_APP=flaskr
 
       4. initialize the database with this command:
 

examples/flaskr/flaskr/__init__.py

@@ -0,0 +1 @@
+from flaskr import app