Add facebook login

[merwok]

Add BaseFacebookUser, subclass of BaseEmailUser.

Make sure updating from class User(BaseEmailUser) to class User(BaseFacebookUser) is painless.

Make facepy dependency optional with extras.

[merwok]

New plan considering #30:

• change model code to use an identity model
• add inline admin class
• move from demo to main package
• no migration required to add facebook login capacity!