Small change.

Adds a density, friction and restitution paramter to the boxWorld.addObjects() method, which allows
for some more control over the box2D model.  These parameters set the (you guessed it) density, friction
and restitution of the object that you want to create.

(4/16/14)
Merge pull request #33 from galaxykate/jpagnutt-b2dAddObject

Small enhancement to the boxWorld.addObject() method