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/
What I meant was that both checks in sync_group_create ('if group does not exist', and 'if user does not exist in group') don't have a lot of value imo.
You should be able to confirm it works without that logic (ie. if the group exists, `synogroup --add ${GROUP_SYNC}` doesn't break anything, and if the user is already in the group, `addgroup ${USER} ${GROUP_SYNC}` doesn't break anything either) You could probably add `> /dev/null` to the addgroup command, but I don't think it shows output usually.

On Diaouls proposal, my take is that there are hardly compatibility issues to be expected, because you don't remove any existing permissions. Great, less chance of breakage ;)
Correct me if I'm wrong, but from what I can see, with this PR, users can set permissions via the `users` group, or the new `synocomm-sync` group. Both will have the exact same effect from a btsync package perspective. The main difference is that with the new group, users will be able to set permissions for btsync on a certain folder without allowing all members of the `users` group access to that folder (and maybe it's easier for people to understand how to set access...). 

@Diaoul Is there a different reason to add the check you suggested? We wouldn't want to halt the install/upgrade process halfway through, would we?