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 I'll flip-flop on this one and saw it's fine to have more options. I agree about it being easier and I do want monban to be extremely configurable. So now the questions is about names again or it's about whether or not another layer dealing with authorization can be developed that would encompass these necessities and push them out of Monban all together. I think I might spike out what that would look like and then decide. If it creates too much overhead and makes Monban + this other thing a burden to use then it might make sense to put it into Monban even if it wades into authorization a little. I'm hoping this won't be the case.