Initial working version of libfreenect2

Hi,
I wanted to discuss, whether we want to start working on first official version of libfreenect2. @floe and me got the internals working. So there is RGB and depth decoding. Depth decoding still lacks a bit of performance. What's basically missing is a C API exposing the functionality we've developed so far. 

I think it's good at this stage to continue the development here in the official repo. Also to attract more contributors. What do you think?

Christian