New node: move_base_straight/move_base_straight.py

This is an action server using the move_base_msgs/MoveBase interface.
Uses the base laser to avoid collisions when driving forward, but driving backward is still unsafe.
move_base_straight: small doc fixes
move_base_straight: renamed goal_topic param
move_base_straight: made footprint_frame + laser_frame params
move_base_straight: add check if scan received