http/hostname set as :: does not work
I have the following config:

``` [mpd]
hostname = ::

[local]
enabled = true
media_dir = /media/equinox-music/
playlists_dir = /home/lasse/Music/playlists
tag_cache_file = /home/lasse/.config/mopidy/tag_cache

[http]
enabled = true
#hostname = 0.0.0.0
hostname = ::
port = 6680
static_dir = /home/lasse/dev/mopidy
```

When I start mopidy with this, I get:

```
INFO     2014-06-27 16:43:41,945 [8860:MainThread] mopidy.commands
  Starting Mopidy frontends: HttpFrontend, MpdFrontend
DEBUG    2014-06-27 16:43:41,946 [8860:Thread-9] mopidy.http.actor
  Starting HTTP server
DEBUG    2014-06-27 16:43:41,947 [8860:Thread-9] mopidy.http.actor
  Loaded HTTP extension: mopidy
INFO     2014-06-27 16:43:41,947 [8860:MainThread] mopidy.mpd.actor
  MPD server running at [::]:6600
DEBUG    2014-06-27 16:43:41,948 [8860:Thread-9] mopidy.http.actor
  Loaded HTTP extension: lux
DEBUG    2014-06-27 16:43:41,948 [8860:Thread-9] mopidy.http.actor
  Loaded static HTTP extension: lux
DEBUG    2014-06-27 16:43:41,948 [8860:Thread-9] mopidy.http.actor
  Loaded static HTTP extension: moped
DEBUG    2014-06-27 16:43:41,948 [8860:Thread-9] mopidy.http.actor
  HTTP routes from extensions: [(u'/mopidy', <class 'mopidy.http.handlers.AddSlashHandler'>), (u'/mopidy/ws/?', <class 'mopidy.http.handlers.WebSocketHandler'>), (u'/mopidy/rpc', <class 'mopidy.http.handlers.JsonRpcHandler'>), (u'/mopidy/(.+)', <class 'mopidy.http.handlers.StaticFileHandler'>), (u'/mopidy/', <class 'mopidy.http.handlers.ClientListHandler'>), (u'/lux', <class 'mopidy.http.handlers.AddSlashHandler'>), (u'/lux/lux/playlist', <class 'mopidy_lux.router.Playlists'>), (u'/lux/lux/loved', <class 'mopidy_lux.router.Loved'>), (u'/lux/lux/cover', <class 'mopidy_lux.router.CoverArt'>), (u'/lux', <class 'mopidy.http.handlers.AddSlashHandler'>), (u'/lux/(.*)', <class 'mopidy.http.handlers.StaticFileHandler'>), (u'/moped', <class 'mopidy.http.handlers.AddSlashHandler'>), (u'/moped/(.*)', <class 'mopidy.http.handlers.StaticFileHandler'>), (u'/(.*)', <class 'mopidy.http.handlers.StaticFileHandler'>)]
DEBUG    2014-06-27 16:43:41,956 [8860:MainThread] mopidy.listener
  Sending playlists_loaded to BackendListener: {}
INFO     2014-06-27 16:43:41,958 [8860:Thread-9] mopidy.http.actor
  HTTP server running at http://:::6680
```

As you can see, the MPD front-end binds correctly, but the HTTP one does not.

diff --git a/mopidy/http/actor.py b/mopidy/http/actor.py
index 5a11c3aa..af12c782 100644
--- a/mopidy/http/actor.py
+++ b/mopidy/http/actor.py
@@ -44,7 +44,8 @@ class HttpFrontend(pykka.ThreadingActor, CoreListener):
     def _startup(self):
         logger.debug('Starting HTTP server')
         self.app = tornado.web.Application(self._get_request_handlers())
-        self.app.listen(self.port, self.hostname)
+        self.app.listen(self.port,
+                        self.hostname if self.hostname != '::' else None)
         logger.info(
             'HTTP server running at http://%s:%s', self.hostname, self.port)
         tornado.ioloop.IOLoop.instance().start()
['mopidy/http/actor.py']