Use a configurable responder when not signed in

The aim here is to to give Monban users full control over what happens when
`requires_login` is run and a `user` is not signed in.

* Default redirect location configurable with `requires_login_redirect`.
 This accepts anything that can be passed to `redirect_to`.
* A configuration for `requires_login_responder` was added. In the
 default configuration, this is a lambda that conforms to the interface
 of ActionController::Responder.

Using the responder interface seemed a good way to offer flexibility for doing
more advanced things (responding appropriately based on MIME types, for
instance).
> I think either we have to implement a solution similar to the one proposed or remove the before_filter all together since it itself represents a form of authorization. 

Checking if someone is authenticated, while somewhat authorizy, certainly falls well within the scope of an authentication library. I still think we should not "skirt" the authz vs auth question, else we open the door for future scope creep and become the problem we're trying to solve.