Spaces around operators.
Export the PhysicsEngine constructor instead of an instance.
Pull in lodash.
Moved hard-coded values into PhysicsEngine.prototype as defaults, eliminated PhysicsEngine#_setup(), and added `options` argument.
Added PhysicsEngine#onPostStep() to handle the cannon.js 'postStep' event and propagate it to all bodies.
Replaced PhysicsEngine#onPostStep() with generic dispatchToAllBodies(), and added propagation of 'preStep'.
Added TargetVelocityController class.
Updated index.js to export the right things.