Support writing tags over sftp

Picard works great with local filesystems. I've tried to use it through the remote server feature of gnome, and it could find files, help we update their tags, but I couldn't save the modifications.

Here's the error that was triggered when picard tried to save metadata:

Feb 11 19:38:01 rsertelon-desktop picard.desktop[24651]:     f = open(filename, 'rb+')
Feb 11 19:38:01 rsertelon-desktop picard.desktop[24651]: IOError: [Errno 95] Operation not supported: '/run/user/1000/gvfs/sftp:host=xxxxx,user=xxx/home/xxx/music/path_to_file.mp3'

I used picard this way because I store my music collection on my dedicated server with ampache for web access and subsonic app on my phone.