Add a destroy function on the viewer to clean up and remove elements
created by open seadragon. Add removeAllHandlersForAllEvents to clean up all events on destroy. Clear
the onDraw callback on Overlay destroy.
fix: calling viewer.destroy() multiple times throw an error. And a simple destroy to mousetracker
fix: comment updates and add a simple test for viewer.destroy
update the comments and unit test
Merge pull request #179 from lukemurray/destroy

Add a destroy function on the viewer to clean up and remove elements