Support for debug draw

[Boscop]

It would be very useful to have support for debug draw.
As far as I understand it, only this class has to be wrapped, and SetDebugDraw(), right?
https://github.com/rjanicek/liquidfun-rust/blob/master/liquidfun-cpp/Box2D/Common/b2Draw.h

[Boscop]

I just added support for debug draw in my local fork, but I still have to test it..