Refactor to `app/handlers/` and `app/models.py`

Instead of cluttering the root of the project, I prefer to put my `RequestHandler` and `Model` subclasses into modules and packages. In practice, I follow a very POSIX-inspired project layout with an `app/` folder that contains all the code that I'm going to send to GAE:

* app/
  * app.yaml
  * dispatch.py
  * lib --> ../lib/python2.7/site-packages/ (generated by `pip install`)
* bin/
* etc/
  * requirements.txt (generated by `pip freeze`)
* lib/
  * python2.7/site-packages/ (generated by `pip install <package>`)
* var/