Fixed missing Event in SpriteSheetBuilder_MovieClip example.

Signed-off-by: Grant Skinner <info@gskinner.com>
Added EventDispatcher.on() and .off(). Fixed an issue with extending EventDispatcher. Added Event.remove().

Signed-off-by: Grant Skinner <info@gskinner.com>
DisplayObject, SpriteSheet, and SpriteSheetBuilder to extend EventDispatcher. Removed redundant EventDispatcher init in Sprite.

Signed-off-by: Grant Skinner <info@gskinner.com>
Updated DragAndDrop example to use .on()

Signed-off-by: Grant Skinner <info@gskinner.com>
Updated libs and docs.

Signed-off-by: Grant Skinner <info@gskinner.com>