added a heartbeat function running over the io-streams.
fix: fixes issue #11.

I implemented a heartbeat in the connection establishing function
'connection' in Mtgox.Pipes.Server.Live. The heartbeat is done via a
thread that awakes every so and so many seconds. I'd prefer a solution
where the heartbeat is handled in the socketio pipe. However, the
information of the heartbeat frequency is only available in the
connection function.