added OrderNotFound -2013 to binance errors fix ccxt#2317

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added exx fetchOrderBook limit fix ccxt#2426

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restored py/php files (missing async anyway) fix ccxt#2425

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kucoin trade side dealDirection → direction fix ccxt#2409

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gdax ohlcv default limit 350 → 300 fix ccxt#2422

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added decimal_to_precision constants to ccxt module

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moved huobipro orderbook nonce parsing to fetchOrderBook

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fixed a bug in numberToString (was unable to handle negative numbers …

…in scientific notation) + added a test for that function

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added more corner case tests for decimalToPrecision + a fix for those…

… cases

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fixed fee cost parsing @ bitfinex

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