Fix claiming of partially claimed items

Looks like I accidentally checked for `quantity_claimed` in the item
template rather than `quantity_remaining`.

I'm also taking this opportunity to adjust `quantity_remaining` to
always return a positive quantity for items with an unlimited quantity.
Use Factory Boy for test factories

Rather than reinventing the wheel for the factories used to generate
test objects, Factory Boy has the ability to do everything I was already
doing and then some. It even hooks into SQLAlchemy.

It's being added to both the tox and Travis CI setups as well as all the
test cases.