Transport: Add a method to close a connection explicitly.

Zab needs to be able to close connections to peers that don't respond
to heartbeat messages. This patch adds a disconnect() method to the
Transport which closes the underlying connection to the peer and
clears any pending outgoing messages.
Implement broadcast in Transport.java.
Add netty-based transport.
Merge pull request #34 from m1ch1/netty

Implement a netty-based transport