Renamed anchorsAuto to mAnchorsAuto for consistency
Made it possible to change Joint.bodyA/bodyB

Changing either body of a joint will now cause the joint to be
re-created. Unsetting either will cause the Box2D joint to be destroyed.

The 'mouse' example has been simplified accordingly.
Added b2JointDef reference to Box2DJoint base class

This allowed introducing a Joint.type property and using
b2JointDef::collideConnected directly instead of having
mCollideConnected. The latter also made sure that collideConnected
defaults to true for the PulleyJoint, as it does in Box2D.