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
Reset currentScene after animated scene transition

When using scene transitions that do not "match", e.g. opening
a scene with "slideRight" and closing it with "crossFade", the
original scene was sometimes invisible because it was still outside
the screen or completely translucent.
Merge pull request #85 from theGoodEvil/reset_scenes_after_animation

Reset currentScene after animated scene transition
#84 Fix Button:setFontName()