Update btsync to 1.4.91 and added group btsync in DSM, for easy folder permissions

1. Updated to 1.4.91 (No changelog at the moment: http://forum.bittorrent.com/topic/32059-latest-desktop-build-1491/)
2. Group btsync is created, which is visible in the DSM. So the user can give folder permissions for btsync easily.