ENH: Implemented AUTO_INITIALIZE feature.

Created a new flag in TradingAlgorithm that enables subclasses to
decide if they want to handle setting self.initialized = True.
Before it was the responsibility of an overriding subclass to set
initalized = True. This was causing problems because it's easy to
forget this. Now it is the responsibility of TradingAlgorithm
unless explicity stated otherwise.

tests/test_algorithm.py

@@ -608,7 +608,7 @@ def handle_data(context, data):
     def test_order_in_init(self):
         """
         Test that calling order in initialize
-        will return an error
+        will raise an error.
         """
         with self.assertRaises(OrderDuringInitialize):
             test_algo = TradingAlgorithm(