Add EthAccount support

[andrew-fleming]

This PR is a tmp working example to support EthAccount txs through Nile. In order to setup an EthAccount:

1. Deploy normal account
2. Compile and declare EthAccount
3. Deploy EthAccount through regular account
   • pass eth address as arg
4. In accounts..json, manually set the passed eth_address as the key and the EthAccount deployment address i.e.

{
  "<ETH_ADDRESS>": {
    "address": "<DEPLOYED_ETHACCOUNT_ADDRESS>,
    "index": 0,
    "alias": "<ETH_ACCOUNT_PRIVATE_KEY>"
  }
}

5. Send a tx with eth-send and the private key alias i.e. nile eth-send PKEY my_contract increase_balance 123