Clear_costmap_recovery - Different parameter syntax

The old behavior was to specify a string parameter, and any obstacle layer with that string within its name would be cleared. 

The new behavior is to specify a list of string names, and any costmap layer with a name that matches one on the list is cleared. 

Fix for #211. Possible fix for #198. 

Includes tests!
Might want to be keep the "to unstuck robot" part of the message, since a lot of ros-users questions reference this specific phrase.