OpenNI2 Driver fixes+improvements

This PR includes several fixes and improvements for the OpenNI2 device driver:

- Cleanup of legacy methods from old driver (Kinect)
- Gracefully close ports and quit if sensor is not attached and input file for playback is not specified
- Add parameter for setting minimum confidence (default = 0.6)
- Add ONI files playback and record functionality (ONI files: OpenNI format for recording/playing back RGBD sessions)
- Add parameter for looping when playing back an ONI file (default = false)
- Update verbose information
- Add README file that points to wiki.icub.org/OpenNI2 with installations and usage instructions (page is being updated)

I have squashed trivial commits and rebased my fork, so this PR should hopefully be clean and tidy :).

Many thanks to Giovanni Saponaro for testing the driver and for suggestions.