Made Box2D.h reference less specific

Our experience with trying to create a [CocoaPod](http://cocoapods.org/) for b2dJson is that `#include <Box2D/Box2D.h>` causes issues because under CocoaPods Box2D is found under `#include <box2D/Box2D.h>`.

Removing the folder reference fixes this for us, and we hope this will work with other users of b2dJson, so long as their compiler searches for headers recursively.