Add limit price parameter for stop loss orders

[benibienz]

Currently, when you create an order with place_stop_order() you create a stop limit order where the limit price is equal to the stop price.

This PR adds an optional argument limit_price to place_stop_order(), allowing the user to specify the limit as well as the stop price.

Note that our limit_price argument is equivalent to the API parameter price, whereas our price argument maps to the API parameter stop_price. This is a little confusing but I wanted to leave the default price arg of the function unchanged to maintain backwards compatibility.

[mabergerx]

This would be nice to have in the live version!