Add new init method which returns the generated value instead of embedding it 

Based on this [SO question](http://stackoverflow.com/questions/27417061/reaching-jwt-token-in-play2-silhouette-jwtauthenticator) it would be nice if the authenticator could return the generated value instead of embedding it into the the response or request.

The JWT authenticator should then return a string representing the generated JWT, the cookie authenticator should return a `Cookie` instance, the session authenticator a `Session` instance and so on.