Added World.autoClearForces and World.clearForces()

When autoClearForces is set to false, the user will have to manually
call clearForces() when it is desired that forces are cleared.

Also made the World.step() function invokable from QML, in case somebody
would want to manually step the world after setting World.running to
false.
Create the b2World in Box2DWorld constructor

Avoids some if conditions and allows directly using the b2World instance
for storing its properties.
Merged Box2DDestructionListener into Box2DWorld

No need for signal connection and simpler in general.
Made tbe b2World a direct member of Box2DWorld

Doesn't need to be a pointer member.
Added property group to access profiling information

Could be useful during development.

This is only the raw profiling information collected by Box2D itself, it
does not include the overhead of qml-box2d. It would probably be
interesting to measure also the synchronization phase.
Merge pull request #44 from bjorn/master

Box2DWorld improvements, additions and optimizations