THORChain ERC20 token swap support

Examples:

    # List available assets:
    $ mmgen-swaptxcreate -S

    # Create a Tether-to-LTC swap transaction for autosigning, connecting to the
    # swap quote server via Tor:
    $ mmgen-swaptxcreate --autosign --proxy=localhost:9050 ETH.USDT 1000 LTC

    # After signing, send the transaction via public Etherscan proxy over Tor:
    $ mmgen-txsend --autosign --quiet --tx-proxy=etherscan --proxy=localhost:9050

    # After sending, check the transaction status and receipt:
    $ mmgen-txsend --autosign --verbose --status

    # Create a Tether-to-DAI swap transaction, with explicit destination account:
    $ mmgen-swaptxcreate ETH.USDT 1000 ETH.DAI E:01234ABC:3

Testing:

    $ test/cmdtest.py -e ethswap

MANIFEST.in

@@ -16,6 +16,7 @@ include test/*/*.py
 include test/*/*/*.py
 include test/ref/*
 include test/ref/*/*
+include test/ref/*/*/*
 include test/ref/*/*/*/*
 include test/overlay/fakemods/mmgen/*.py
 include test/overlay/fakemods/mmgen/*/*.py