Various baxter.sh updates

Allows sourcing baxter.sh script.
Adds argument 'local' for running on robot.
If ROS_IP/ROS_HOSTNAME not set, 'local' or 'sim' set ROS_HOSTNAME to 'localhost'
Reinserting ability to specify other baxter_hostname (ROS_MASTER_URI) as argument.
Merge pull request #160 from RethinkRobotics/baxter_sh_update

Various baxter.sh updates