Move Broadphase back to a real interface. Should be fine.
Added DebugDraw for physics2d, and the physics2d_callbacks
sample from turbulenz samples (slightly modified as its mapping
table isn't actually used and not needed for sample).

Fixed bug on Shape::addEventListener/removeEventListener wrappers
to ensure that a 'null' mask (no undefined in Haxe) is passed as
'undefined' to the JS function.