Move Notifier class to lib/rollbar/notifier.rb. Added Rollbar.scoped.

* Rollbar.notifier returns a Notifier instance by thread
* Rollbar constant delegates the public interface to a Notifier instance.
* Added Rollbar.scoped. It changes the payload_options inside the received block.
* I've cleaned some code and some tests in this commit.
Use new Rollbar.scoped method in Rails middleware.