rtt_roscomm: improving robustness

- added correct exported targets behavior
- made it so that typekits should be generated before calling
  catkin_package
- fixed problem with re-setting catkin_INCLUDE_DIRS when the orocos
  cmake package was found during initialization in
  GenerateRTTROSCommPackage.cmake.em (it's still unclear why this was
  re-setting catkin_INCLUDE_DIRS)
- updated unit test