Possible trade info improvements

[gadfly16]

Hi,

I think TradeResponse.Last shoud be a string and TradeInfo.Time should be a time.Time. Now both are typed as int64 and I think it's less than ideal.

For some time now Kraken trade response includes 4 digits of fractions to the trade time. This data gets lost when we convert it to int64.

Also, for me it seems Kraken intends the last value to be used as it is in the next request, so converting back and forth is unnecessary and possibly not very future proof. It will overflow in april of 2611, but Kraken can also change its content anytime, so I think it would be better not to presume that it can be represented as an int64.

If it makes sense, I'm more than happy to make the pull request fixing these issues.

Best regards,
Mate