Openni2 pullrequest

Experimental version of an OpenNI 2.x grabber. 

I tested the existing OpenNI 1.x grabber under VS2010 and did not find any obvious regressions or errors, so the two should be able to coexist (as long as they are not built at the same time). There is a CMake check preventing this.

Several apps/examples break encapsulation and directly use OpenNI 1.x data structures, so they will not work with the new grabber until this is fixed.

Linux compatibility has not been tested, but the new grabber will likely fail until FindOpenNI2.cmake is updated with the correct default paths.

This branch also includes some minor fixes related to building with Visual Studio 2012.