Add rosbuild support in GenerateRTTRosCommPackage and rtt_ros service

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.