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.
Fixed abstraction around existing OpenNI 1.x grabber.

This should help with switching between OpenNI 1.x and 2.x grabbers.
Added fall-through to OpenNI 2 grabber.

If OpenNI 2 is being used, have openni_grabber.h include openni2_grabber.h.
Enabled kinfu CMake option with OpenNI 2.
Added OpenNI2 library checks when building kinfu app.
Disabled some kinfu classes that depend on OpenNI 1.x when OpenNI 2 is being used.
Masked oni_grabber errors when the OpenNI 1.x wrapper is not used.
Added conditional compile instead of changing OniGrabber cast type.

Should prevent regression problems when using OpenNI 1.x.
Disabled pcl_record_tsdfvolume when using OpenNI 2.
Refactored obscure kinfu app function names.
Fixed confusing names in kinfu app.