Implements basic models support.

As you said, not having models pretty much defeats the purpose of a single-file framework, so I hacked together a basic implementation. 

Basically, I hijack the metaclass-magic, django uses to load models and mock application and models modules.

Some design decisions:

* both application models and `django.db.models` are accessible through `d.models`. May be somewhat confusing, but I found it quite intuitive
* Models access is case-insensitive (like in django `get_model`)

I updated `foo.py`, go to `/edit/` and enter 2 values, which are saved in the DB on post. `/index/` will then list values of every saved object.

There is convertion support, the example app doesn't work after convertion, unfortunately. There is some fhurl-magic in lines 250-252, I wasn't sure how to handle properly. I'll open a separate issue for it.

EDIT: you'll have to run `python foo.py syncdb` first to create a DB, off course.