Sender should allow to retrieve payment status

[gip]

From the documentation (e.g. the README, see below), one can easily infer that when the callback is called the payment is successful. I don't think the current implementation enforces that -- tests showed that in some condition the payment didn't complete (source account amount was decreased by the payment amount + fee but the destination account wasn't updated -- the money was held in escrow). We should make sure this is clear and maybe the return value should be a promise with a payment status?

    send({
      sourceAccount:      'http://localhost:3001/accounts/alice',
      sourcePassword:     'alice',
      destinationAccount: 'http://localhost:3002/accounts/bob',
      destinationAmount:  '1',
      notary:             'http://localhost:6001',
      notaryPublicKey:    'QD/UBKyptEXcu6mZThsfnE/2ZZGsrpokKqaLMUrTUqo=',
      //receiptCondition: { message_hash, signer, public_key, type },
    }).then(function() {
      console.log('success')
    })