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).
Looks like this was originally going to be a responder, but is now a proc instead?