Adds extra constraint of year to check for day inequality.

The .day member of datetime only gives the number of the day of the
year, which is insufficient when checking for < than.

However in this use, we may just want to check for an 'not equals'

zipline/finance/slippage.py

@@ -83,7 +83,9 @@ class VolumeShareSlippage(object):
             if(order.dt < event.dt):
 
                 # orders are only good on the day they are issued
-                if order.dt.day < event.dt.day:
+
+                if (order.dt.year, order.dt.day) < \
+                   (event.dt.year, event.dt.day):
                     continue
 
                 open_amount = order.amount - order.filled