Add test factories

These factories -- `xmas.events.models.Event` and
`xmas.users.models.User` are currently supported -- can be used to
generate instances of models for testing.

Each instance will automatically be added to the session. Just make sure
the factory is called inside a request context.
Return the claim when claiming an item

The subject pretty much says it all. It might be useful to have the
claim instance after making the claim.

Some commented out code is also being removed.
Add tests for `Item.claim`

Normally I like to try to commit all of a model's tests together, but I
must walk away from the computer now and I'd like to have the tests for
`xmas.events.models.Item.claim()` preserved. This is another important
method to get under test.

When I return to writing tests I need to address the `TODO` about
`item.quantity_claimed`. It doesn't appear to be updated correctly. I'm
not sure yet if it's failing to update in the database or if it's the
instance isn't updating correctly. The update does, after all, go
directly to the database.