Add OpenNI2 driver
Improve Makefile. Add KF_DRIVER and allow setting the compiler from outside
Improve argument parsing
Add --replay option to replay .oni file.

For now this is only supported for OpenNI2.
Allow simultanous recording of what the camera sees.

The recording is saved to an .oni file.
This currenlty only works with the OpenNI2 backend.

In combination with --replay, this allows us to reproduce results offline.