Flask factory pattern support

[KeNaCo]

Lib don't allow to use factory pattern.

[Informatic]

Well, dirty hack for that is:

social._state = social.init_app(app, WhateverDatastoreYouImagine(...))

Oh, and, btw, it's properly fixed in #70

[KeNaCo]

Already working on it.
I'm not fun of dirty hacks.. and #70 is way away from proper fix.. self._state is nice shortcut to object if you still have instance, but proper way is using app context.