catkin_generate_changelog fails for mercurial repo

fails when running `catkin_generate_changelog` for mercurial packages with following error:

~~~
Found packages: multisense, multisense_bringup, multisense_cal_check, multisense_description, multisense_lib, multisense_ros
Querying commit information since latest tag...
Traceback (most recent call last):
  File "/usr/bin/catkin_generate_changelog", line 120, in <module>
    main()
  File "/usr/bin/catkin_generate_changelog", line 104, in main
    tag2log_entries = get_forthcoming_changes(vcs_client)
  File "/usr/lib/pymodules/python2.7/catkin_pkg/changelog_generator.py", line 68, in get_forthcoming_changes
    latest_tag_name = _get_latest_version_tag_name(vcs_client)
  File "/usr/lib/pymodules/python2.7/catkin_pkg/changelog_generator.py", line 93, in _get_latest_version_tag_name
    tag_name = vcs_client.get_latest_tag_name()
  File "/usr/lib/pymodules/python2.7/catkin_pkg/changelog_generator_vcs.py", line 286, in get_latest_tag_name
    cmd_log = [HgClient._executable, 'log', '--rev', '.', '--template', '{latesttagdistance}']
AttributeError: type object 'HgClient' has no attribute '_executable'
~~~

Most likely a typo, where `HgClient._executable` should have been `self._executable`.
Merge pull request #102 from hsu/patch-1

catkin_generate_changelog fails for mercurial repo