Send transaction via Etherscan

[AndreMiras]

Since we currently can't connect to other nodes using pydevp2p (refs #5), we could publish the (locally signed) transaction to Etherscan.

[AndreMiras]

Almost done, I think we're just missing the nonce calculation.
And then a lot of error management and UI polishing :)
And by the way the etherscan API (eth_sendRawTransaction) for transaction broadcasting is not very talkative. On wrong nonce this is what I got:

{u'status': u'0', u'message': u'NOTOK', u'result': u'Error!'}

But sending that same transaction hex on https://etherscan.io/pushTx I got this:

{"jsonrpc":"2.0","error":{"code":-32010,"message":"Transaction nonce is too low. Try incrementing the nonce.","data":null},"id":1}

[AndreMiras]

Fixed in recent commit. Will do the polishing in dedicated tickets