Missing dependency on python-xdg

Running classicmenu-indicator without python-xdg installed results in the following error:

Traceback (most recent call last):
  File "/usr/bin/classicmenu-indicator", line 9, in <module>
    load_entry_point('classicmenu-indicator==0.09', 'console_scripts', 'classicmenu-indicator')()
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 356, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2476, in load_entry_point
    return ep.load()
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2190, in load
    ['__name__'])
  File "/usr/lib/python2.7/dist-packages/classicmenu_indicator/__init__.py", line 22, in <module>
    from cmindicator import main
  File "/usr/lib/python2.7/dist-packages/classicmenu_indicator/cmindicator.py", line 35, in <module>
    import about
  File "/usr/lib/python2.7/dist-packages/classicmenu_indicator/about.py", line 7, in <module>
    from settings import vars as settings
  File "/usr/lib/python2.7/dist-packages/classicmenu_indicator/settings.py", line 5, in <module>
    import xdg.BaseDirectory
ImportError: No module named xdg.BaseDirectory

The attached patch adds the missing dependency.