OpenNI2-FreenectDriver

[OpenNI2-FreenectDriver](https://github.com/piedar/OpenNI2-FreenectDriver) has existed for a while as a separate project.  It allows OpenNI2 to use libfreenect as a Kinect driver since Kinect is not natively supported on non-Windows platforms.

This pull request will merge it into libfreenect.  This vastly simplifies the installation process as OpenNI2-FreenectDriver may now be easily linked with a static libfreenect.