enhancements to deal with mulit-robot-szenarios by using name spaces for...

Hello

I added some check of the nodes name spaces to deal with multi-robot scenarios. It would be nice if you accept my changes. 

Greetings
Markus

Details:
Name spaces are working perfect to rename topics but most nodes do have shared parameter to name a topic to which should be subscribed and than the problem start :-(
But I solved the problem and my amcl and move base is working now smoothly.
I forked the navigation stack and added a check on shared parameter like amcl: odom_frame_id, base_frame_id, global_frame_id to get the namespace right.
I did the same for the the following nodes
amcl/src/amcl_node.cpp
costmap_2d/plugins/obstacle_layer.cpp
costmap_2d/src/costmap_2d_ros.cpp
dwa_local_planner/src/dwa_planner.cpp
fake_localization/fake_localization.cpp
move_base/src/move_base.cpp

It would be nice if you could accept my pull request, if so I will right short tutorial how one can deal with multiple robots.