Modified to match current Turtlebot Indigo version
Modified to use arlobot version of navigation parameter files instead of turtlebot versions.
Integrated PING sensors into costmap obstacle avoidance. Also added some delays to serial startup and shutdown to avoid leaving the Activity Board in an unknown state. Also added code to save and restore last reported X, Y and Heading so we do not lose our place if we restart propeller_node while roscore stays up. This could also be used to load an X, Y and Heading from the launch file if we want to.