Merge python-bitcoinrpc into python-bitcoinlib

RPC calls are now in bitcoin.rpc module, specifically the bitcoin.rpc.Proxy() class. This class automatically generates the proxy URL from ~/.bitcoin/bitcoin.conf and is intended to eventually get rid of the need to mess around with JSON serialization and deserialization.

I left the full bitcoinrpc history intact because those who forget history are doomed to repeat it, or something.

Also fixed Python3 compatibility.