make nosetests --xunit-file argument an absolute path to work around nose bug 779 (fix #659)
add imported library targets for find_package'ed gtest

This makes catkin gtests work again with gtest 1.5 as well as
1.6+ on Arch Linux and others.
fix handling of cmake packages which do not install any files (fix #665)
escape messages to avoid CMake warning (#667)
Reducing likelihood of race condition in .catkin marker file

- Uses atomic (<4k write) file(APPEND ...) call to update .catkin marker
  file with less likelihood that parallel catkin processes interfere
  with each-other.
- Only potential for a race condition exists if the .catkin marker file
  has not been created yet. In that case, the tool running the multiple
  instances should check this file and either report the failure in a
  clear way and suggest building again, or simply repair the file,
  itself.
catkin_generate_environment: don't overwrite existing .catkin file

- During install step of a catkin package, don't overwrite an existing
  .catkin file
- During a devel build, don't append a semicolon-prefixed source path to
  the .catkin file if the .catkin file is empty. This is related to the
  previous race condition fix which prevented overwriting the .catkin
  file. This should introduce negligible sensitivity to parallel
  invocations of catkin-based CMake builds only if the .catkin file is
  empty. For all other cases, it is still an atomic <4k APPEND write.
avoid using ARGN for efficiency
fix regression of #676
update changelog
0.5.89