OpenNI2 driver: proper Kinect support and improvements

In this PR:

* Proper Kinect support. Libfreenect (with the OpenNI2-freenect bridge) is needed for Kinect support in OpenNI2. Until recently, this bridge was very temperamental, hence the reason why Kinect was not supported by the YARP driver. Latest versions have fixed the stability issues. With this PR, the YARP driver detects if the sensor is a Kinect or not, and behaves accordingly.
* The sensor name is displayed in the output (PS1080 for Primesense-compatible devices, Kinect for KinectV1
* Added support for more color and depth modes for Xtion sensors (recent firmware upgrades improved the capabilities of the sensors)

Testing and support of KinectV2 coming soon (hopefully!)

Documentation is being updated at http://wiki.icub.org/wiki/OpenNI2

Commits rebased :)