Emitting events - analytics

@martynjames 

I added handlers that are called via AJAX calls when the iframe or image is loaded. I would like if someone from the analytics team take a peak at this too see if it works as expected.
I've always used ``self.runtime.publish`` from within XBlocks. Do we need to add this as a reference for this XBlock - right now it depends upon the container runtime installing ``eventtracking`` - which it does, so I think we're okay, but something to think about.