hitbtc2 cleanup

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added support for since and limit in fetchTrades to Bitfinex v2 ccxt#760



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luno ratelimit 1 request per 10 (!) seconds

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new endpoint for exchangeInfo added to binance

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minor edits to coingi

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added coingi fetchMarkets, fixed fetchTickers

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bithumb fetchMarkets fix (transpiled from JS to Python / PHP) ccxt#758

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minor typos

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minor edits to kraken

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added extra headers to js base class

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