Bullet, update to 2.81

This includes https://github.com/libgdx/libgdx/pull/462.

- Update Bullet to version 2.81 rev 2613
- Separate the src folder into bullet and custom, the src/bullet folder now contains an exact copy of bullet source, with two exceptions:
- btRaycastVehicle includes some code that swig doesn't like, for the fix see https://github.com/xoppa/libgdx/commit/a33818a7fa227345785e61b14f560e5c6a6a5e00.
- Remove BulletMultiThreaded and MiniCL (which are only useful for PS and XBOX)
- Split the SWIG interfaces into Collision, Dynamics and Softbody.
- Update the build scripts, visual studio project, etc.

What's changed: http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?t=8490
Tested on win64 and android.