Implement the possibility to automatic generate a stair urdf models

[GiulioRomualdi]

This PR introduces the possibility to generate an urdf model of the stair. The model is automatically generated when the cmake is configured.

In details:

1. generate_stairs_urdf.py implements a script for automatically generate an urdf model of a stair
2. GenerateAndAddUrdfModel.cmake implements the machinery for automatically generate the urdf model.

This is an example of the automatically generated model:

[diegoferigo]

If you're using gazebo you can also use erb to script a sdf file. You can have a look to the recently added tutorial: https://github.com/ignitionrobotics/ign-gazebo/blob/ign-gazebo3/tutorials/erb_template.md.