cmd_vel to 0 when deadman button is released
When deadman is not pressed publish once a zero twist to make the robot stop. Publisher is now latched as commented in https://github.com/turtlebot/turtlebot_apps/pull/75#issuecomment-33087125
Merge pull request #75 from martimorta/hydro

cmd_vel to 0 when deadman button is released