DEV: Adjust performance calculations for capital changes

Refactor PerformancePeriod so that it creates a sub-period every
time a capital change happens within the period

zipline/algorithm.py

@@ -383,6 +383,9 @@ class TradingAlgorithm(object):
 
         self.benchmark_sid = kwargs.pop('benchmark_sid', None)
 
+        # A dictionary of capital change values keyed by timestamp
+        self.capital_changes = kwargs.pop('capital_changes', {})
+
     def init_engine(self, get_loader):
         """
         Construct and store a PipelineEngine from loader.