Add retrieval of focal length from camera and publishing it

So far the kinect server did not provide access to the focal length of the camera. However, this is needed in some libraries. Therefore this PR adds the possibility to retrieve the focal length from the camera (both, using KinectSDK and OpenNI) and publish it such that users have access using the KinectClient.

Let me know if there are any concerns. I tested this in Linux as well as Windows and it works fine.

Best,
Tobias