Figure out a better way to integrate robot descriptions

[GianniLunardi]

At the moment the robot description is compiled on the fly using a generic xacro with other xacros with properties for each robot.

This doesn't work when we load the RVIZ model meshes bc it doesn't work by pointing to the mesh file directly, it wants the package the meshes belong to.

To fix that, in #4 we now expect a ${ROBOT_TYPE}_description package to be present, just for the meshes. We should make the system to either use the official robot description or internally create fake packages, to avoid external dependencies.

[mcamurri]

I am making a more accurate CAD with rails that will be added to this repo:
https://github.com/idra-lab/aliengo_description_old

The name "old" is bc @mfocchi replaced it with his own, but we should converge to only one robot description.