Fixed UnicodeEncodeError in `OGDSActor.get_link`

Traceback:
```
  Module opengever.dossier.viewlets.byline, line 38, in get_items
  Module opengever.dossier.viewlets.byline, line 18, in responsible
  Module opengever.ogds.base.actor, line 83, in get_link
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 8: ordinal not in range(128)
```

All actors `get_link` method should return unicode, so it makes sense to make this change.