Check whether body is created in two Box2DBody methods

Otherwise calling these methods on the wrong moment would crash.
Simplified synchronization code

Rely on synchronizing the position and angle of the b2BodyDef to avoid
doing unnecessary conversions and to make the checking of changed
values accurate, removing the need for qFuzzyCompare.
Use itemChange rather than connecting to rotationChanged

This could be a little faster, but I didn't measure it. I mainly changed
it because it's more similar to how we adjust to position changes.
Use invertY helper function for World.gravity

Just makes the code a little shorter.
Merge pull request #45 from bjorn/master

Synchronization simplifications