Fix DOM leak for bkCell by unregistering handlers on destroy
Fix DOM leaks for bkCodeCell
Fix DOM leaks for output cell menu by unregistering handlers on destroy
Fix leaks in EpicEditor and markdown cells
Make sure session is cleared when switch from mainapp to control-panel
Reset impl handle in mainapp on bkNotebook destory in preventing leaks
Fix memory leak in bkOutputDisplay by ensuring child scope is destoryed
Fix a memory leak by clearing cells array properly when reseting notebook model
Do not make bkDebug a global variable by default.
Debugging can be turned on by bkHelper.debug()
Add debug UI method to bkDebug
Fix memory leak caused by unregistering window.onbeforeunload
Fix memory leak by unregistering connected status listener
Fix memory leak by unsubscribing cometd on bkNotebook destroy
Fix memory leak by unregistering handlers on bkNotebook destroy
Merge remote-tracking branch 'origin/master' into alee/fix-266
Merge pull request #400 from twosigma/alee/fix-266

Fix #266, memory and DOM leaks