Depth scaling parameter

This pull request adds a parameter for depth scaling, called "z_scaling". Together with the already present parameter "z_offset" it is possible to "correct" depth values using a linear function of type `y=ax+b`. Instead of full stereo-like calibration one can use these parameters to compensate for bad (factory-) focal length calibration of the IR camera.
The following graph shows depth measured using a checkerboard detection & solvePnP in the RGB image (mean z of all checkerboard corners) on the y axis and the corresponding depth of the depth image on the x axis together with a fitted line for one of our Asus XtionPRO Live cameras.

![Screenshot from 2013-04-15 16:08:53](https://f.cloud.github.com/assets/1481786/383688/98abe296-a621-11e2-9892-30e0126a4ca2.png)