Merge pull request #7 from cdondrup/master

Works fine on Linda and also addresses one of the message generation issues.
Added implementation of cmd_vel_mux to allow arbitration. To use run scitos_teleop_mux.launch and have the navigation publish to /cmd_vel_mux/input/navigation instead of /cmd_vel.
Joystick will now get priority over navigation if dead-man-switch is pressed.
Added README file with basic instructions.
Updated package.xml to make rosdep work.