Updated Bullet and DirectX Toolkit to current

Also, some fixes to everywhere:
- Multithreading support (using C++11 threading model of course!), with
asynchronous operations
- Better input management, using the multithread support (every key
stroke or mouse event call an asynchronous function)
- Attempting to apply multiple morphs to a material
- Code refactoring everywhere
- Skyboxes!
- Physical world stubs - the Bullet Physics objects are created, but
they are not updated on the renderer, although we have a physics
simulation running on background
- Many other improvements that I can not remember