set useCapture true to be consistent with Animation
Return a removal function when adding an event listener.
Add an EventHelper class.

This makes it easy to remove any number of event listeners on any number of events when destroying an instance.
Merge pull request #892 from AnalyticalGraphicsInc/event

Event improvements
Merge branch 'master' into geojson
Changes after review.
Merge branch 'geojson' into dynamicScene-cleanup