Reset cached event objects

The Event's source, target and callback could hold references to
objects that are not needed otherwise, preventing them from being
garbage collected.

Events are cached in both EventDispatcher.EVENT_CACHE and
InputMgr.TOUCH_EVENT.
Reset cached TouchHandler.TOUCH_EVENT
Merge pull request #83 from theGoodEvil/reset_cached_events

Reset Cached Event Objects
small update
Merge branch 'master' into dev