fix #84: roslocate broken for old distros, also metapackage not recognized

get_cached_release raises RuntimeError for all distro names <= fuerte, need to handle.
Detecting metapackage export manually, not sure whether library function exists somewhere, did not see any in catkin_pkg

Modified unit tests to test the new function
small message to notify ROS_DISTRO is being used
fixed help display of roslocate
Merge pull request #85 from tkruse/fix_rosdistro_locate

fix #84: roslocate broken for old distros, also metapackage not recognized