Blender Git Loki

Adding the start to an async option for bge.logic.LibLoad(). The basic threading works, but there are still some issues that need to be addressed:
* Memory leaks when canceling a thread. I'll try to fix these with more cleanup callbacks like the one used in async_convert().
* The user has no way of knowing when an asset has been loaded. I'm thinking some sort of Future object would work here.
* Only works for scenes. It will be simple to support Mesh and Action as well, but I want to do it in a such a way as to minimize code duplication.