ENH: Add direct methods for session start/end.

Rather than having to do 'start, _ = cal.open_and_close_for_session(dt)'
to get just the start, we can now do 'start = cal.session_start(dt)'.

tests/calendars/test_trading_calendar.py

@@ -670,6 +670,14 @@ class ExchangeCalendarTestBase(object):
             found_open, found_close = \
                 self.calendar.open_and_close_for_session(session_label)
 
+            # Test that the methods for just session open and close produce the
+            # same values as the method for getting both.
+            alt_open = self.calendar.session_open(session_label)
+            self.assertEqual(alt_open, found_open)
+
+            alt_close = self.calendar.session_close(session_label)
+            self.assertEqual(alt_close, found_close)
+
             self.assertEqual(open_answer, found_open)
             self.assertEqual(close_answer, found_close)
 

zipline/utils/calendars/trading_calendar.py

@@ -643,6 +643,18 @@ class TradingCalendar(with_metaclass(ABCMeta)):
         return (o_and_c['market_open'].tz_localize('UTC'),
                 o_and_c['market_close'].tz_localize('UTC'))
 
+    def session_open(self, session_label):
+        return self.schedule.loc[
+            session_label,
+            'market_open'
+        ].tz_localize('UTC')
+
+    def session_close(self, session_label):
+        return self.schedule.loc[
+            session_label,
+            'market_close'
+        ].tz_localize('UTC')
+
     @property
     def all_sessions(self):
         return self.schedule.index