REP 138 LaserScan Common Topics, Parameters, and Diagnostic Keys

First REP in a series to define standardized topic names, parameter names, and diagnostic keys for various driver groups.  If this one goes well, other types should follow such as GPS, IMU, Camera, and Joystick.

Posting for initial review on ROS SIG drivers on 2/21/2013.

Posted to ROS users on 2/25/2013.