Re-implemented the Calendar API.

Instead of having separate ExchangeCalendar and TradingSchedule objects, we
now just have TradingCalendar.  The TradingCalendar keeps track of each
session (defined as a contiguous set of minutes between an open and a close).
It's also responsible for handling the grouping logic of any given minute
to its containing session, or the next/previous session if it's not a market
minute for the given calendar.

setup.py

@@ -96,7 +96,11 @@ ext_modules = [
     Extension(
         'zipline.data._minute_bar_internal',
         ['zipline/data/_minute_bar_internal.pyx']
-    )
+    ),
+    Extension(
+        'zipline.utils.calendars._calendar_helpers',
+        ['zipline/utils/calendars/_calendar_helpers.pyx']
+    ),
 ]