Refactoring: responder implementation has been moved to a nested module.

That makes it friendly to another useful 'responders' gem, allowing to combine responders:

    responders ActsAsApi::Responder::Module, Responders::Pagination

The change is backwards-compatible, so you still can use original way of changing responder using

    self.responder = ActsAsApi::Responder

No new tests, as no new functionality was added.