Breaking change for API key

[jaggedsoft]

The old version had this format: Client(address, privKey) but the new version has this format Client(apiKey, address, privKey)

If you leave the api key out and use it like the old format (address, privatekey) it sends the private key as the address to idex servers. So people who update the code but don't add the api key get their private key sent to IDEX servers and it returns Invalid address

Maybe do a sanity check for required 3rd parameter on Client instantiation