actionlib_msgs: use the DIRECTORY argument instead of the BASE_DIR argum...

...ent of add_message_files. The DIRECTORY argument of add_action_files now also takes absolute paths.

This hopefully makes the BASE_DIR argument of add_message_files useless. (See https://github.com/ros/genmsg/pull/51).

Sorry that it took so long, I wasn't able to find some time to do this earlier.

Does anyone have a good idea how deactivate packages like sensor_msgs, when installed via dkpg? I would need this feature to test my pull request, since I don't really want to set up a virtual box and compile from source (at least not today).
The `genmsg` version supporting this is `0.5.4`.