Better to use filenames which are commonly used in the python word then
custom ones.

Create a factory module per app and move them under the new
oscar.test.factories module.

This makes it more efficient to load offers now that we can't simply use
the date filters to exlucde unavailable offers.

The is_active field has been reworked to depend on the
get_max_applications method.

The save method of ConditionalOffer now auto-sets status.

Fixes #403