ENH: Add a convenience property for open amount of a blotter.Order

Found that when implementing different slippage model, needed the
open amount in several places/functions, having the open amount
as a property of an order should help reduce the need for passing that
around and maintaining the value separately.

zipline/finance/blotter.py

@@ -261,3 +261,7 @@ class Order(object):
             return False
 
         return True
+
+    @property
+    def open_amount(self):
+        return self.amount - self.filled