Adds a members property to SyncDistribution.

The meta.members property was introduced to allow a community to limit
data dissemination.  When meta.members is an empty set all messages are
synced, however, when meta.members contains one or more Member
instances, only messages created by those members will be synced.  For
example, adding only your own Member instance will disseminate only
0-hop messages.
Merge pull request #2 from boudewijn-tribler/zero-hop-distribution-policy

Adds a members property to SyncDistribution.