Replace bodyB with another bodyB update

VRope-jonnyijapan 0.5.1
I just extended this stuff with what I needed for now. It's still a
work in progress.

Some changes include:

- APP_PTMRATIO needs to be defined somewhere. I define it in the apps's
.pch file, then make use of that when defining PTM_RATIO. (all in one
place)
- I needed to detach the "bodyB" (sometimes called body2) and "leave
the rope hanging", or sometimes attach another body as "bodyB".
replaceBodyOfRopeWithBody... does this.
The way I make the rope "hang" is I create an invisible b2body of a
small size and just attach it. I needed to recreate the joint to make
it work. Make this can be made in a better way. Beware of memory
mistakes.
- Other things I forgot...