Devise strategy - add basic backward compatibility

Reintroduce acts_as_token_authentication_handler_for and acts_as_token_authenticatable
These methods can provide aliases for the new API but also give deprecation warning.

After merging this to ```spike-refactor-concerns-to-devise-strategy``` branch, you should be able to use code from ```spike-refactor-concerns-to-devise-strategy``` branch in your existing application and in 99% of cases it should work :)