Strategy now accepts an authenticate method ref

- To ease integration with either stormpath-rails or stormpath-sdk gems, the authenticator passed to the strategy is now a reference to a method. This would allow the developer to pass a reference to either a custom authenticate method (in the Sinatra case) or (in the case of stormpath-rails) a reference to the "authenticate" method on whatever class they're using to store user information.
Merge branch 'features/accept_authenticate_method_instead_of_class-2' into development