Begins updating feeds

This is again, a rough, early cut.  Blah blah blah.

This commit strips out some stuff from the commafeed models that's
probably unnecessary, and implements a naive feed fetcher.  There's
still some work to come in terms of properly updating feeds, improving
the tests to be more stringent, and refactoring to make more sense.

Next up: updating a feed that already has entries (testing for
conflicts?), and the fever API.  Once that's working, I can deploy to
Heroku and start dogfooding.
Formats the RSS example we had so I can build better tests
Removes one element from the example RSS

This is so that we can have a "reddit-new.rss", which is the same, with
the extra element.  We can then test that the feed updating mechanism is
robust.
Introduces safer feed updating

This is incomplete, however, and doesn't deal with many edge cases.  I'm
also not happy with the .validate_unique() call -- I'd rather do some
sort of .save_or_update (if that existed), but I'm offline right now and
can't check the documentation.

This is therefore a naive implementation that gets the tests passing.
The next step is to make the tests a little more stringent.

Following that, I'm going to make the backend models a little more
pythonic (last_update_date???) and begin work on the Fever API.
Updates Feed model to be a little more Pythonic

Renames updated_on to be refreshed_on, as updated is an overloaded word
in RSS.  Also adds tests that enforce that this attribute is updated.

Finally, this commit also removes a bunch of stuff from the Feed model
that was deemed unnecessary.  The attributes were mostly imported from
Commafeed, under the belief that they would probably have thought this
stuff through better than me.  However, on second thought, a lot of their
advanced features just add weight to the model that I don't need yet --
so it's gone for now, and when we start adding more advanced/robust
features, we can add some of those attributes back.