OpenNI: Depth Sensing Cameras

This adds support for OpenNI 2 compatible depth sensing cameras, such as the Asus Xtion Pro.

## Dependencies

OpenNI 2 SDK: Prebuilt binaries are availble for Windows, and GNU/Linux on Structure.io's website. For Windows, add the 'OpenNI2/Redist/' folder to your PATH environment variable. If you used the default installation location, this will be:

C:/ProgramFiles (x86)/OpenNI2/Redist/