Bounding volumes improvements

- Added implementation for BoudingFrustum.Intersects(BoundingSphere)
- Simplified/Optimized BoundingFrustum implementation

We (Flying Development Studio / Infinite Flight) are in the process of syncing our MonoGame branch to the latest and greatest (we used an old fork from september 2011 that we heavily customized for our needs). A lot of things seem to work right off the box   and i'll be sending pull requests for what we fix along the way in order to get Infinite Flight to work with MonoGame 3.X.