Rename the "get_new" method of pk-fields to "_validate"
RedisModel: Make self.pk.get() do the work previously done by self.get_pk()
Get rid of get_pk

(using _pk in tests)
Raise DoesNotExist instead of ValueError if object not found
Merge pull request #75 from twidi/work-on-pk

Get rid of get_pk, and raise DoesNotExist when needed, instead of ValueError