andbot_gazebo

add range_sensor_layer in costmap so sonar can see obstacles.

Reference

http://wiki.ros.org/range_sensor_layer

http://answers.ros.org/question/206805/adding-range_sensor_layer-to-layered-costmap-for-global-planning/

https://github.com/corot/thorp/tree/master/thorp_bringup/param/move_base

Issues:

1. .world files path name
2. STL problems