OpenNI2 driver updates and improvements

In this PR:

* Added --printVideoModes option to list all available video modes (resolution and FPS) by the RGB and the depth sensors
* Added --depthVideoMode option to set the mode of the depth sensor
* Added --colorVideoMode to set the mode of the RGB sensor
* Updated driver output accordingly so the user gets info about separate video modes
* Fixes and improvements in the comments

Video mode in this context is resolution and FPS. E.g., 640x480@30
Documentation at http://wiki.icub.org/wiki/OpenNI2 is being updated

Commits squashed and rebased :)