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).
Because that's the public interface for a responder. I thought it would be nice to use that interface here as it's baked into rails and would allow writing something like:

```ruby
class MonbanResponder < ActionController::Responder
  def call(controller, resources, options)
    respond_to do |format|
      format.html { #... }
      format.json { #... }
  end
end
```

which is nicer than having to prefix `respond_to`, `flash`, `redirect_to`, etc with `controller` or having to set up your own delegation. `ActionController::Responder` is already doing that.