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.
documentation: make Indigo intro more like Groovy and Hydro (#440, #656)
Merge pull request #664 from jack-oquin/indigo-devel

documentation: make Indigo intro more like Groovy and Hydro (#440, #656)
fix handling of cmake packages which do not install any files (fix #665)
escape messages to avoid CMake warning (#667)
Merge pull request #668 from ros/escape_messages

escape messages to avoid CMake warning
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.
Merge pull request #671 from jbohren-forks/marker-file-race-condition-append

use atomic APPEND write mode for writing source spaces to .catkin marker file
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.
Merge pull request #676 from jbohren-forks/dont-overwrite-marker-on-install

catkin_generate_environment: don't overwrite existing .catkin file
Merge pull request #666 from ros/handle_cmake_package_without_installing_any_files

fix handling of cmake packages which do not install any files
Merge pull request #663 from v4hn/fix-gtest-with-find_package

add imported library targets for find_package'ed gtest
avoid using ARGN for efficiency
update changelog
0.6.8
add missing test dependency on mock
fix regression of #676
update changelog
0.6.9
refactor code from run_tests.py into Python module