Implemented working alternate storage backend, based on MongoEngine.

Implemented working alternate storage backend, based on MongoEngine.

It doesn't necessarily need to live in the primary codebase, it is simply a proof of the original idea. With the appearance of a fair number of helper methods to decouple social auth from django models, it really is looking like a backend is in order. More details:

+ Add MongoEngine models
* Add several methods to both models modules to decouple social auth core and default pipelines from Django ORM code
* In social pipelines, make sure uid is a str value
* Only load django admin settings if current models are django models
+ Add a conf module to manage loading specific Django settings and default values to use