Switching back XHR/Image to have no event data.
Until Issue #365 lands.
Cleaning up BaseEventTarget to properly dispatch events.
ReplayProvider now tracks all DOM events (that it knows about).
These are recorded as 'wtf.replay.dispatch#[name]'. The playback system
will use these as its steps to advance replay. There should be enough
information to properly rematerialize those events.

The trickier event types are those relating to objects like XHR/Image/etc.
Those are not yet supported.
Sharing more code between callsites and fixing some misc bugs.