Support multiple networks via config file

[HarryR]

Currently the proof of concept only uses a single Ethereum JSON-RPC connector, and assumes that either side is operating on the same contract on the same network for testing purposes.

To be useful it needs to support multiple networks on both the client & coordinator server side.

I think this should be done via a config file, e.g.:

networks:
  eth-main:
    type: ethereum
    id: 1
    connector: ethjsonrpc
    url: http://127.0.0.1:8545
  eth-ropsten:
    type: ethereum
    id: 3
    connector: ethjsonrpc
    url: http://127.0.0.1:8546

Other information that could be contained in a config file:

• Token addresses, which network they're on
• HTLC and other swap contract addresses
• Approved contract code hashes

This also opens the possibility of using Infura or Etherscan to submit transactions, instead of a direct EthJsonRpc connection, as long as they both provide the same interface.

Will need to find a clean method for supporting the config file in the client & server, e.g. when creating a connection it will use an object factory from the config class.