MAINT: Moves internal state variables in performance tracker.

Slight refactoring of grouping the tracking variables in the
PerformanceTracker together.

So that it's easier to see which are config members and which are
members used to track internal state.

zipline/finance/performance.py

@@ -157,14 +157,8 @@ class PerformanceTracker(object):
         first_day = self.sim_params.first_open
         self.market_open, self.market_close = \
             trading.environment.get_open_and_close(first_day)
-        self.progress = 0.0
         self.total_days = self.sim_params.days_in_period
-        # one indexed so that we reach 100%
-        self.day_count = 0.0
         self.capital_base = self.sim_params.capital_base
-        self.returns = []
-        self.txn_count = 0
-        self.event_count = 0
         self.cumulative_risk_metrics = \
             risk.RiskMetricsIterative(self.period_start)
 
@@ -193,6 +187,13 @@ class PerformanceTracker(object):
             serialize_positions=True
         )
 
+        self.returns = []
+        # one indexed so that we reach 100%
+        self.day_count = 0.0
+        self.txn_count = 0
+        self.event_count = 0
+        self.progress = 0.0
+
     def __repr__(self):
         return "%s(%r)" % (
             self.__class__.__name__,