ENH: Make RollingPanel update itself if new fields arrive.

Before we preinitialized the BT's fields and sids.
Thus, no new ones could be added after initialization.
This should be fixed now.

tests/test_batchtransform.py

@@ -43,7 +43,7 @@ def return_price(data):
 
 
 class BatchTransformAlgorithmSetSid(TradingAlgorithm):
-    def initialize(self, sids):
+    def initialize(self, sids=None):
         self.history = []
 
         self.batch_transform = return_price(
@@ -111,15 +111,14 @@ class TestChangeOfSids(TestCase):
         )
 
     def test_all_sids_passed(self):
-        algo = BatchTransformAlgorithmSetSid(self.sids,
-                                             sim_params=self.sim_params)
+        algo = BatchTransformAlgorithmSetSid(sim_params=self.sim_params)
         source = DifferentSidSource()
         algo.run(source)
-        for df, date in zip(algo.history, source.trading_days):
+        for i, (df, date) in enumerate(zip(algo.history, source.trading_days)):
             self.assertEqual(df.index[-1], date, "Newest event doesn't \
                              match.")
 
-            for sid in self.sids:
+            for sid in self.sids[:i]:
                 self.assertIn(sid, df.columns)
 
             last_elem = len(df) - 1