Implement emerald_sign

[gagarin55]

Method which allows sign arbitrary message.

The sign method calculates an Ethereum specific signature with: sign(keccak256("Ethereum Signed Message: " + len(message) + message))).
Parameters

    Address - 20 Bytes - address.
    Data - Data which hash to sign.

params: [
  "0xcd2a3d9f938e13cd947ec05abc7fe734df8dd826",
  "0x5363686f6f6c627573" // Schoolbus
]

Returns

    Data - Signed data.

[r8d8]

We need to add passphrase into rpc call, to unlock keyfile for selected adress