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.
Since we are not building a library, why do we need to export all these dependencies? Another package using find_package() to find this package probably just wants to use the messages, in which case many dependencies are unneeded (e.g. cv_bridge, image_transport, artoolkit, ...)