Dataframe resampling by groups
The examples that I see for [`resample`](http://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.DataFrame.resample.html) seem to only involve `pd.Series`, although the method is technically defined for a `pd.DataFrame`.

Can it be used to do the type of more complex resampling described [here](http://stackoverflow.com/questions/33662514/aggregation-every-x-days-in-pandas)? Can I perhaps do this combining `resample` with `transform` having it work on one-column at a time as in the examples with the `series`?
