Use a configurable responder when not signed in

Use a configurable handler 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 `no_login_redirect`.
  This accepts anything that can be passed to `redirect_to`.
* A configuration for `no_login_handler` was added. This allows the
  developer to easily completely customize the response to account for
  MIME types or whatever they please.

sign_in_notice might need to change as well. `no_login_notice` or `no_login_message` seem appropriate, but I'll save that for another PR.