* Allow unsafe code with BulletSim
* Tweak the BulletSimN API a bit.
* Dumping BulletSimNPlugin in favor of combining the API
This updates prebuild to remove BulletSimN, implements the BulletSim API in BulletSPlugin using the BulletXNA Bullet physics engine.  It also updates the BulletXNA library to be compatible with the changes.   OpenSimDefaults has been updated to describe how to switch engines and terrain implementations.