Added C++11 flag for GCC.

For GCC version:
< 4.3:  warning
< 4.7:  -std=gnu++0x (and warning)
>= 4.7: -std=c++11

A separate flag is not needed for Visual Studio, as support is solely dependent on the compiler version.
Fixed vague naming.
Fixed CMake typo.
Added optional dependency check.

Added:
* WITH_OPENNI2

Allows building without OpenNI 2.x, even if it is available on the system.
Started refactoring of OpenNI 2 grabber.