Results of HRI code sprint for stereo-based road area detection (stereo module)

Here is the first part HRCS' results (only for the stereo module).

Add in module stereo:
* DisparityMapConverter class for converting disparity maps to point clouds.
* DigitalElevationMapBuilder class for building Digital Elevation Maps from disparity maps.

Add in module common:
* A PointDEM point type for representing Digital Elevation Maps.
Return values should be enclosed in brackets, see [PCL style guide](http://pointclouds.org/documentation/advanced/pcl_style_guide.php) (everywhere in your code)