existing `limit_price` and `stop_price` parameters. The goal of this change is
to refactor the existing ordering API to provide a cleaner interface for
defining more complex order types.
Adds a new module, zipline.finance.execution, which defines the ExecutionStyle
abstract base class, along with concrete MarketOrder, LimitOrder, StopOrder,
and StopLimitOrder subclasses.
Adds a new `style` keyword argument to the function signature of the `order`
API method, which accepts an instance of ExecutionStyle.
The existing limit_price and stop_price parameters are still supported at this
time, but are converted into the new ExecutionStyle objects before being passed
to Blotter.order.
Scott Sanderson