Added full pre-emption and getting the current planner velocities from move_base instead

This pull request does several things:

* Adding pre-emption for move_base and in general
* Resets point cloud publishing etc when pre-empting, resets PTU
* Adding a behaviour that does a sweep, this is not supported in scitos_2d_navigation atm
* Returning 'success' if local planner fails, 'failure' if global planner fails
* Going backwards slower now (0.2)
* Launch the required nodes (`republish_pointcloud_service` and `previous_positions_service`) together with monitored_navigation