Accept strings in group_by

[dbrgn]

I think it should be possible to set the group method in group_by by passing a string.

Right now, I'm using: import djangosphinx.apis.current as sphinxapi query(q).group_by('ch_id', sphinxapi.SPH_GROUPBY_ATTR, '@weight desc')

Goal: query(q).group_by('ch_id', 'SPH_GROUPBY_ATTR', '@weight desc')