abstracting Stage to allow for social network or portal submissions of FlashPunk games

FP and Engine both bind to Stage during initialization which can interfere with website and social network API's that need to do the same.

FP.stage is modified to DisplayObject to allow for you to pass in a given DisplayObject into the Engine constructor. I made it an optional parameter in order to preserve existing FlashPunk games and to avoid the community chasing me with pitchforks.

Note that this @CaffeineViking deserves all the praise and glory if this fix helps you out. All I've done is abstract it into a pull request for the rest of the community.

(reference: http://dev.playvue.com/2013/07/using-root-instead-of-stage-in-flashpunk-haxepunk/)