Pull request for issue 3

I'm afraid the issue 3 has returned..

```
Traceback (most recent call last):
  File "/usr/bin/spotify-dbus.py", line 295, in change_listener
    self.show_playing()
  File "/usr/bin/spotify-dbus.py", line 137, in show_playing
    self.nid = interface.Notify('Spotify', 0, self.get_cover(), self.get_info(track, 'title'), text, actions, hints, self.timeout)
  File "/usr/bin/spotify-dbus.py", line 417, in get_cover
    os.system('wget -q -O ' + path + ' ' + match.group(0))
AttributeError: 'NoneType' object has no attribute 'group'
```

I'm using the gentoo version equivalent to 20130119 .

Interrestengly, most of the time cover fetching works and only sometimes fails.
I worked around this issue it in the pull request.