Add OpennNI 2 driver

I added OpenNI 2 support, which allows using KFusion with the Asus Xtion and other Primesense devices.

I also included a few other fixes and changed the Makefile so that the C++ compiler, include paths etc. can be set from outside the Makefile and don't need to be hardcoded.

My OpenNI error handling is rigorous but verbose. Tell me if you would prefer a macro instead to make calls+checks one-liners.