Add rosbuild support in GenerateRTTRosCommPackage and rtt_ros service

[meyerj]

This is a follow-up of #9. Not all goals mentioned there have been achieved yet, especially these two points:

rosbuild packages can find_package(rtt_roscomm) and call ros_generate_rtt_typekit() and ros_generate_rtt_service_proxies() on rosbuild message packages. This replaces the former rosbuild_include(rtt_rosnode GenerateRTTtypekit) call.

The rtt_ros service should be able to import rosbuild packages.

[jbohren]

Looks good!

[meyerj]

There is still some more work to do here, especially the rosbuild support in the rtt_ros service. I just wanted to collect all patches here. I could look into it next week.

[jbohren]

Yeah, though I haven't tested it yet, I think since the rtt_ros service uses the rospack C++ API, as long as the manifest.xml files have similar <export><rtt_ros><plugin_depend> tags, it will work exactly the same way. I'm going to respond to all of the other comments from Ruben and Peter over the next couple of hours.

[jbohren]

We should probably re-vist the naming/design of those export tags too, since that's still open: #3