New RayCast class

That's all around b2Fixture::RayCast (b2RayCastOutput *output, const b2RayCastInput &input, int32 childIndex). QML/JS not allows to pass arguments by reference if I understand it right so I decided to push it to different object. It may be useful to get distance to nearest shape and whatever. I tried to keep names according to Box2D API. Also I created small exampe to test it