Load social_auth models based on a configuration setting

* This provides the ability to supply alternate storage modules
* Moved models into django_models module, and gave each model an app_label
* Defined a configuration setting, SOCIAL_AUTH_MODELS, that defaults to the django models
* Still need to define the protocol for implementing new storage modules
v0.7.0
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
Merge remote-tracking branch 'upstream/master' into alt-storage
Added new keyword argument for developers to override the OAuth default http method ("GET").
Made new parameter last for compatibility for positional arguments.
Merge pull request #374 from czpython/master

Added new "method" parameter
Move get_user method to specific models backends
Moved definition of USERNAME_MAX_LENGTH to models modules
Merge remote-tracking branch 'upstream/master' into alt-storage
Replace raw ORM query for User by email with new models module method
Replace user-or-id resolution with new models module method
In context-processor, replace type-check with check for auth method
Replace raw queries of social auth by uid/provider, and social auth user creation with new methods in models modules
Replace raw queries/creates in DjangoOpenIDStore with new model module methods
Fix github new app link
Merge branch 'master' of github.com:omab/django-social-auth
Handle set_expiry overflow error.
Merge pull request #378 from sayar/fix-tz-overflow

Handle set_expiry Overflow Error.
Fix typo in configuration docs
"The function parameters will..." -> "The function parameters are..."