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/
The only reason is to keep track on when this change has been done (which SPK_REV) in the code. I know it's harmless without it but it makes things a bit more standard with an easier to maintain in the future if we finally decide to add some other changes.
See https://github.com/SynoCommunity/spksrc/blob/develop/spk/nzbget/src/installer.sh#L97