Add support for adjusting how far back in history the trending strategy will go

Author: maihde

strategies/trending.py

@@ -66,7 +66,12 @@ def __init__(self, start_date, end_date, initial_position, market, params, h5fil
             self.addIndicator(symbol, "rsi", RSI(rsi)) 
 
         # Backfill the indicators 
-        d = start_date - (30 * ONE_DAY)
+        try:
+            backfill = params['backfill']
+        except KeyError:
+            backfill = long_
+
+        d = start_date - (backfill * ONE_DAY)
         self.updateIndicators(d, start_date)
 
     def evaluate(self, date, position, market):