title: Kraken-API OHLC request doesn't honor the 'since' parameter
body: <p>I'm trying to retrieve 3 months history of 30-minute candles from Kraken using the REST API:
  <a href="https://www.kraken.com/help/api" rel="noreferrer">https://www.kraken.com/help/api</a></p>
  
  <p>Following the documentation, I made this POST request to URL:
  <a href="https://api.kraken.com/0/public/OHLC" rel="noreferrer">https://api.kraken.com/0/public/OHLC</a></p>
  
  <p>passing the following parameters in JSON format:</p>
  
  <pre><code>{
    "pair":"EOSETH",
    "interval":30,
    "since":1514404800
  }
  </code></pre>
  
  <p>where 1514404800 corresponds to the timestamp (in seconds) to the date:<br>
  27/10/2017 20H00m UTC<br>
  This is not clear from the API documentation, they call it an 'ID', but I came to this conclusion by looking to the returned values.</p>
  
  <p>So I was expecting returned response with entries starting on this date. Then I would fetch subsequent entries using the last id returned.</p>
  
  <p>However the first entry I get corresponds exactly to 15 days ago. Actually, if I don't pass the parameter 'since' at all, I get exactly the same result, so it seems the parameter is being ignored completely.</p>
  
  <p>Maybe Kraken changed the API and this parameter 'since' was replaced by some other? <br>
  Or I missunderstood the syntax of this parameter and I'm doing something wrong?</p>