Fixes issue backtrader2#61: self.positions throws attibute error when IB Broker used.

self.positions = collections.defaultdict(Position) is consistent in all the other brokers, was missed in IB.

Author: neilsmurphy

backtrader/brokers/ibbroker.py

@@ -276,6 +276,7 @@ def __init__(self, **kwargs):
         self.ordstatus = collections.defaultdict(dict)
         self.notifs = queue.Queue()  # holds orders which are notified
         self.tonotify = collections.deque()  # hold oids to be notified
+        self.positions = self.ib.positions
 
     def start(self):
         super(IBBroker, self).start()