make RPC the default
pass a message router to up/download
make SendMessage a method of RPCServer
make zeronetwork implement MessageRouter
update server for RPC
RPC overhaul of State

Using RPC alters how State operates in a few key ways. The good
news is that these changes almost uniformly result in cleaner,
simpler code. state_test.go is a little uglier because type
assertions are required now, but that's about the only down side.
update consensus for RPC
remove TCP code
remove returncodes from HandleSignedHeartbeat
use literal in CreateState()
Merge pull request #100 from lukechampine/rpc

Make RPC the default MessageRouter