Link pthread as private

This fixes the CMP0023 warning with cmake 2.8.12
Disabled warnings when compiling 'diff' library on Windows
Change status type from int to nite::SkeletonState in all getSkeletonState(int) methods
Quit OpenNI2DeviceServer if sensor cannot be found or cannot be initialized
Remove bool argument from updateInterface() (obsolete concept from OpenNIv1.x)
Remove getSkeletonState() method
Clean up unused commented code
Add parameter for minimum confidence
Add information for the verbose parameter
Comments cleanup