Added the on arg to resample on a grouped dataframe.

Author: Nicholas Ver Halen

pandas/core/groupby.py

@@ -1140,13 +1140,13 @@ def ohlc(self):
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
-    def resample(self, rule, *args, **kwargs):
+    def resample(self, rule, on=None, *args, **kwargs):
         """
         Provide resampling when using a TimeGrouper
         Return a new grouper with our resampler appended
         """
         from pandas.tseries.resample import get_resampler_for_grouping
-        return get_resampler_for_grouping(self, rule, *args, **kwargs)
+        return get_resampler_for_grouping(self, rule, key=on, *args, **kwargs)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)