Changes EventWindow parameter of days to window length.

The days parameter was causing confusion when explaining the
usage of the batch_transform decorator.

tests/test_algorithm.py

@@ -76,5 +76,5 @@ class TestTransformAlgorithm(TestCase):
         assert 'mavg' in algo.registered_transforms
         assert algo.registered_transforms['mavg']['args'] == (['price'],)
         assert algo.registered_transforms['mavg']['kwargs'] == \
-            {'days': 2, 'market_aware': True}
+            {'window_length': 2, 'market_aware': True}
         assert algo.registered_transforms['mavg']['class'] is MovingAverage