DEPR: pd.Grouper key -> on
xref https://github.com/pandas-dev/pandas/pull/15326#issuecomment-278008177

- when ``pd.Grouper`` was introduced, we used ``key`` to indicate the column which to group on
- ``.resample`` added the same idea as ``on``
- we use ``on`` in ``pd.merge`` (existing for a long time)
- ``.groupby`` uses ``by`` (though to be fair this can take lots of things and is not strictly a string column name).

So the proposal is to deprecate ``pd.Grouper(key=...)`` in favor of ``on=``.

