Update btsync to 1.4.93 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.

Testing: http://www.andreasgiemza.de/btsync-for-synocommunity/1.4.91-packages-btsync-group.for-easy-folder-permissions-test/
A check is not really necessary. The sync_group_create first checks if the group exists and only when it doesn't it creates it. When the group already exists and the sync user isn't a member it adds the user.

Sorry, I doesn't really get what you two want.