Added initial scribe implementation

* Adds a group communication layer to BOPlish
* Supports create/subscribe/publish API
* Still needs a timeout mechanism to cleanup the subscriptions (as
described in scribe paper)
Implemented Scribe leave

Subscribers can now leave groups
Just return the damn array
Added initial Group Comm. interface to protocol

Protocols gain a `group` object that hooks up the scribe group
communication
Issue periodic subscription refreshs

* Subscriptions have to be actively refreshed or they time out
* Create messages are also periodically routed to the rendezvous point
as the Chord implementation currently does not do any key maintenance
Fixed leave

LEAVE messages are now correctly propagated up the distribution tree