Fixed abstraction around exiting OpenNI 1.x grabber.

This should help with switching between OpenNI 1.x and 2.x grabbers.
Added OpenNI 2 check when building kinfu.
Added OpenNI2 library checks when building kinfu app.
Added OpenNI 1.x/2.x switches.
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.