Support multiple variations of similar robots

[adam-slucki]

As we'll potentially have multiple robots of the same kind it should be possible to support all of them with minimal changes in the codebase.

1. The launch files should be parameterized.
2. URDF descriptions should be created with xacro macros.
3. Messages returned by sensors could include additional information so the nodes responsible for acting upon them would;t have to know about the specifics of the robot (i.e. that the robot XXX sensor "a" is mounted 10 cm above ground on the front of the robot, etc.).
4. Ideally if multiple robots could use the same hardware interface.
5. Possibly the structure of some nodes should change, as well as the names of directories, etc.