MAINT: refactoring for orders api

- moved Order and Blotter to zipline.finance.blotter
- moved order method from AlgoSimulator to Blotter
- eliminated the set_order method in algorithm
- moved blotter to the algorithm
ENH: added cancel function

- removed vestigial methods
- removed code that drops filled orders from memory
ENH: new order management methods:

- get_open_orders
- get_order
- cancel