Integrate Alchemy with Spree.user_class

When adding alchemy_spree, the presumption is that that there is
a Spree store, which already uses an authentication system and the
user class is referred by Spree.user_class.

Also fixes logic in User Decorator and eliminates decorating complexity.

Adds suggestion in README on integrating the Alchemy pages with the Spree layout
without adding magic to the engine.

Tested with Spree 3.0 and Alchemy 3.0