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.
> If ar_pose depends on those components, they should be added as dependencies.

You need to differentiate a bit more. These are dependencies that are needed for building the package, but they are not needed by other packages using our package.

That's why these dependencies are in the `package.xml` as `build_depend` (so they get cmake'd first when you run `catkin_make`), but they are not _exported_ in the `CMakeLists.txt` using `CATKIN_DEPENDS`.
There is no rule like "Every dependency in `package.xml` has to appear in `CATKIN_DEPENDS`".
So, I see no problem there.

Introducing an `ar_pose_msgs` package would break the API, so I'd like to keep that separate from my catkinizing pull request and let @LucidOne decide on it.