headers for OpenNI 'Natural Interaction' frameworks

 OpenNI is a framework for getting data to support 'Natural Interaction',
 i.e. skeleton tracking, gesture tracking, and similar ways of getting data
 from humans. OpenNI provides the interface for physical devices and for
 middleware components. The API enables modules to be registered in the OpenNI
 framework, which then produce sensory data. OpenNI also allows selection of
 different hardware and middleware modules.