Fixing crashing of joints on initialization

Since the world is not known during initialization but only once things are being created, the joints also do not know the value of pixelsPerMeter during initialization. Currently, because `mWorld` is still 0, most joint examples are crashing on startup after #51 got merged.

The solution implemented here delays the creation of a `b2JointDef` instance (along with any necessary conversions) until the point where the joint is actually being created.

Only the `MouseJoint` has been fixed so far, the other joints would have to be adapted in the same way.