robot_state_publisher support array of URDF strings

[mikepurvis]

Would be great if either a) the node could be run multiple times (with a namespaced robot_description parameter), or b) the node could accept multiple strings and unify them to a single large model.

Previous external efforts:

Compose urdf by globbing together xacros into a file of include tags.

Compose urdf using yaml files defining known accessories and a GUI tool.

[sloretz]

@mikepurvis This issue is now located here ros/robot_state_publisher#77