Fix classloader warnings on exit of move_base

Planners were not being properly unloaded, this commit fixes the following warnings on exit:

```
[amcl-2] killing on exit
[map_server-1] killing on exit
Warning: class_loader::ClassLoader: SEVERE WARNING!!! Attempting to unload library while objects created by this loader exist in the heap! You should delete your objects before attempting to unload the library or destroying the ClassLoader. The library will NOT be unloaded.
         at line 94 in /tmp/buildd/ros-hydro-class-loader-0.2.3-0precise-20140110-0815/src/class_loader.cpp
Warning: class_loader::ClassLoader: SEVERE WARNING!!! Attempting to unload library while objects created by this loader exist in the heap! You should delete your objects before attempting to unload the library or destroying the ClassLoader. The library will NOT be unloaded.
         at line 94 in /tmp/buildd/ros-hydro-class-loader-0.2.3-0precise-20140110-0815/src/class_loader.cpp
shutting down processing monitor...
... shutting down processing monitor complete
```