follow_planner: Initial commit

This was forked from navigation/global_planner
It probably doesn't work as is. I haven't tried.
follow_planner: Now it builds

I can start hacking seriously now.
Merge branch 'follow-orig' into follow-planner
follow_planner: add follow goal listener
follow_planner: add logic to follow a moving goal
follow_planner: fix publishing of the plan

Also, fix node prefix for the follow_goal topic