Fixed the collision handling a bit, added "bopping" to kill the bad guys
I think bopping is working pretty well now

Need to remove the tween from the FLY - this is causing some problems with collision detection
Also consider keeping 'static' and 'movable' objects separate so that static collisions can be checked after movable and thus receive priority in positioning
Changed FLY to use its own custom "tween" rather than tweenjs