MAINT: Use a for loop for main algorithm run loop instead of list

So that stepping through a debugger is a little easier, with
respect to having easy access to the algorithm object, and seeing
which step in `self.gen` the interpreter is currently at.

zipline/algorithm.py

@@ -298,7 +298,9 @@ class TradingAlgorithm(object):
 
         # loop through simulated_trading, each iteration returns a
         # perf dictionary
-        perfs = list(self.gen)
+        perfs = []
+        for perf in self.gen:
+            perfs.append(perf)
 
         # convert perf dict to pandas dataframe
         daily_stats = self._create_daily_stats(perfs)