Add callback param to oauth request handler, allowing async calls

This works well for my scenarios.

However, I'm not sure if a) There's some edge cases I should be considering or b) if the value returned from _call should get some special treatment with the async method.  

Seeing as it's handing off to a fairly bullet proof Meteor.http.call, I figure it's probably good, but would be happy to revisit if there's anything you think warrants more attention.