Modified the xacro and launch sctructure to make it more generic:

 * The same xacro and dae file are used for any road (no need to create new ones).
 * It is only necessary to specify the path and the name of the road image and its size.
 * The descritpion launch has been integrated into the spawn_road launch.
 * An example launch file has been added.
Removed the materials.xacro because it was not used.