Adding gate.io integration

[benmarten]

Hi, thanks for you contribution... Would you mind adding:

• a simple test in this fashion: https://github.com/benmarten/CryptoETF/blob/master/test/model/integrations/testBinanceWallet.js
• a readme entry in this fashion: https://github.com/benmarten/CryptoETF/blob/master/README.md#exchanges
• an entry in settings.example.json: https://github.com/benmarten/CryptoETF/blob/master/settings.example.json#L36

Thank you. Happy to merge afterwards :)

[Guiiks]

Hello! I made the changes, could you tell me if it's ok please? Thanks, Guillaume

[benmarten]

Yes, great thank you :)