Adjusting README to show how to avoid issue #80

Special thanks to @ricardodovalle for helping us on it.

Signed-off-by: Rodrigo Oliveira <rodrigo@deadbodyoutline.com>
Refactored to support qml-box2d's transition to qboject based bodies
and target properties.  Added PhysicsEntity and some Box2D Body/Fixture
convienence components.  Entity and Sprite no longer inherit from Box2DBody.