Using nodelets and image-based background subtraction

This release makes the entire code way faster by using ros nodelets.
Also the background subtraction is done on the image side now.

Warnings:

• The realtime_urdf_filter part is not working with Kinect_v2 sensors
• Not working if using openni2 to run the sensors (mm to meters conversion to do)

Multi kinect tracking works

This is the first release.
The documentation has not been updated yet but the code works!