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/
I'd even suggest to remove the current if block from R26 onwards in `sync_group_create`, and then add the conditional with the check on SPK_REV. Those commands (`synogroup --add` and `addgroup`) report a proper error if it can't be executed, but won't break anything, and overwriting the desc wouldn't hurt anyway.