Adding support for PhantomX Pincher gripper

Pulled gripper section of arm.urdf.xacro out into a new file, gripper.urdf.xacro
Renamed existing config files to add prefix, "turtlebot_"
Added similar versions of the config files for the Pincher gripper with prefix, "pincher_".  
Hopefully this paradigm should make it easy to add other grippers or arm configurations.
I've tested with Indigo and Pincher arm - all demo items seem to work fine.  
For Pincher, this requires new version of Arbotix_ROS to support prismatic joint, which I'm sending to Fergs.
Needs testing with original turtlebot arm.