Add if and unless filter options

[gonzalo-bulnes]

As a developer
In order to be able to scope token authentication in a fine-grained way
I want the :if and :unless ActiveSupport callbacks options to be supported by acts_as_token_authentication_handler_for

Example:

# app/controllers/some_controller.rb

acts_as_token_authentication_handler_for User, :only =>[:show], unless: lambda { |controller| controller.request.format.html? }

See #155