package.xml and CMakeLists.txt modifications

Hi, 
I have done some modifications:

package.xml:
 - roscpp and catkin are no required as build_depend.
CMakeLists:
- removed roscpp from carkin COMPONENTS
- added catkin_package DEPENDS
- added a "hack" to allow the rosmsg tool to locate the messages genererated, so that external scripts can
  locate them when adding messages to a node.
  The ROS community has no clean solution to this in thecurrent version.

Compilation pocedure still working perfect, i.e.:
- catkin_make -DCATKIN_WHITELIST_PACKAGES="artoolkit"  
- catkin_make -DCATKIN_WHITELIST_PACKAGES="ar_pose"  

Please, ask a new pull request to LucidOne (original repo).

Thanks, 

Àngel.
Can you explain why this hack is needed? This looks very ugly.
rosmsg finds the .msg files and the generated python code just fine on my system.