daily_order on buy_limit_order and sell_limit_order

[adrianwhite]

V2 of the Bitstamp API has the option of specifying that the buy_limit_order or sell_limit_order can be a daily_order (self cancelling at 00:00 UTC).
Would it be possible to add this in the same way as limit_price is defined?

From Bitstamp API documentation:

daily_order(Optional) | Opens buy limit order which will be canceled at 0:00 UTC unless it already has been executed. Possible value: True

I'll try to submit a PR for this soon (similar to #30)