Proposed revision to event system:
  - EventReceiver no longer exists
  - Instead, objects may define an EventHndler attribute which manages a set of EventEmitter objects
  - Each EventEmitter manages a list of callbacks.

Revised CanvasBackend API to use uncommon names. This prevents name collisions with GUI toolkit classes, thus allowing simpler CanvasBackend subclasses.
Merge branch 'master' of github.com:pyvis/pyvis

Conflicts:
	pyvis/event.py