updated box2d physics sample

hi,

I tried the chipmunk physics sample and was disappointed to not find similar functionality in the box2d sample. The thing is I struggled a bit to get the mouse joint working, so I guess it could be useful to include in the sample how to manipulate an object. What do you think?

this is the first time I do a pull request, so please bear with me if I did stupid things :)

greets
matthias