BUG: Don't include out-of-range date in early closes

Don't include New Year's Eve 1999 in the early closes returned to the
user if it's outside the range of dates requested by the user.

zipline/utils/tradingcalendar.py

@@ -346,6 +346,8 @@ def get_early_closes(start, end):
     # http://www.nyse.com/pdfs/closings.pdf
     #
     # New Year's Eve
-    early_closes.append(datetime(1999, 12, 31, tzinfo=pytz.utc))
+    nye_1999 = datetime(1999, 12, 31, tzinfo=pytz.utc)
+    if start <= nye_1999 and nye_1999 <= end:
+        early_closes.append(nye_1999)
 
     return pd.DatetimeIndex(sorted(early_closes))