setVertexBuffer refactor + tests!

Don't think there's anything controversial here. Moved to the enumerations with the PrimitiveType and modified the setVertexBuffers signature.

I think its more intuitive if the offset is an optional parameter. I also made it so you can give a List<VertexBuffer> or VertexBuffer. I'm okay with having two methods setVertexBuffer and setVertexBuffers if that's your preference.

If you mention anything about the examples directory still being there :rage4: