WIP catchOn/catchOnMain implementation

It would be convenient to have `catch` implemented for a nicer syntax when attaching just an error handler. (e.g. http://emberjs.com/api/classes/Ember.RSVP.Promise.html#method_catch) However, `catch` is a C++ keyword, and appears to be unusable in RXPromise as a result. Thoughts on an alternative name?

(WIP PR implements `catchOn` and `catchOnMain`)