fixed Publishable url uniqueness by db check in clean metod and fixed category related finder

There are three commits:

First is only PEP8 fixes.

Second  is for category related finder. Example in current version of ella: I have publishble x and x has set category y and is also listed in this category and I want 4 related objects by category and finder gives me only 3 objects although there is many objects (This situation happens only if position of x in listing query set is less than count specification).

Last is for Publishable clean method where is check of uniqueness by category, slug, publish_from for non static objects. There is replaced self.__class__ with Publishable, becouse self.__class__ validate uniqueness for specific content type (for example only in articles query set) and not in Publishable qs and it breaks unique url per publishable.