Consider adding support for other caches than default

[lukmdo]

Currently cacheback uses default django cache without a way to use any other alias then default.

This would be also helpful for testing where some test are fine with django backends.dummy.DummyCache wheres some need something like backends.locmem.LocMemCache. My suggestion is keeping azising behaviour as default but adding cacheback.base.Job constructor param cache

@codeinthehole curious to hear your feedback