Introduce new base auth delegate.

auth-delegates/delegates/base introduces a new abstract interface
that provides default functions for methods needed to complete
fully functional auth delegates.
It also provides a getUser function which returns the
auth-delegates/user singleton (contained on the prototype of the
base delegate in the user property).
Make user property on base delegate private
Merge pull request #25 from Livefyre/user_instance

Introduce new base auth delegate.