Added FindOpenNI2.cmake search module.

*also added a find_openni2 macro to PCLConfig.cmake.in to allow external users of PCL to find the OpenNI 2 libraries. This is a duplicate of the code in FindOpenNI2.cmake.
Created abstract Image interface for OpenNI grabbers.
Added OpenNI 2 grabber.
Added OpenNI 2 viewer app to visualization module.

* Changed default viewpoint for OpenNI 2 viewer app. Needed to look down +Z axis to see point cloud.