ENH: Update ordering API to support new ExecutionStyle class in favor of
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.
ENH: Add style parameters to order API helper methods.

Add `style` parameter to order_value, order_percent, order_target,
order_target_percent, and order_target_value methods.  The style parameter is
forwarded to the underlying call to `order`.
MAINT: Add tests for UnsupportedOrderParameters.

Add a test case in test_algorithms to verify that appropriate exceptions are
thrown if an algorithm makes a call to the order api with a stop/limit price
and a style.