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.
In our Lab (and as far as I know, in many places), we use our own scripts to generate nodes quickly. Those scripts look for the messages if they are required as dependencies. In that cases without these lines, the messages are not found. We've been in contact with the OSRF people and they have no better solution to this. Anyway, it can be removed if you don't like it.