Added B2FlxMultiTouchJoint

[catmanjan]

Usage: add(new B2FlxMultiTouchJoint(B2FlxState));

[WingEraser]

Cool, before I merge this can you check the following:

• pool the B2FlxMouseJoint instead of creating a new one every time mouseDown() is being called.
• kill a mouse joint while dragging a body. This works fine in single touch, but I need to know if this works when you drag multiple bodies and kill the mouse joints.
• ObjectMap instead of ConcurrentHashMap. Libgdx util doesn't have GC.

[catmanjan]

Not sure what point 2 means, I've tested it on my application and it works as expected, the only problem is dragging a single body with multiple touches, I am unsure how it should behave by default.