ENH: Add new order statuses for broker integration
BUG: Clean up handling of reject() and hold() orders
TST: Add tests for behavior of rejected/held orders.

Also made a tweak to the handling of Order.status
for when a held order is filled (partial or full).
ENH: Re-work the usage of _status on Order
Merge pull request #380 from quantopian/broker_order_statuses

ENH: Add new order statuses for broker integration