roslaunch: Adding 'doc' attribute to roslaunch <arg> tags

* This adds a 'doc' attribute to the roslaunch <arg> tag
* This tag is read in and stored in the resolve_dict under 'arg_doc'
* The keys in resolve_dict['arg_doc'] are the argument names, and the
  values are tuples where the first element is None or the doc string,
  and the second element is None or the default value.
* The arg_dump.get_args function still returns a dict, so the return
  value's keys can still be used for a list of arg names.
* A unit test has been added to test_roslaunch/test/roslaunch.test which
  loads the ros_args.launch launchfile and compares the output of
  arg_dump.get_args to the expected output.