Asynchronous Image Loading

Changed the imageDecodingPolicy to ON_LOAD, which asynchornously
decodes the image before calling the complete event. This change
improves the responsiveness of the UI while loading flump pacakges.

See Adobe's documentation for more details:
http://help.adobe.com/en_US/as3/dev/WS52621785137562065a8e668112d98c8c4df-8000.html