Added GROUP BY via group_by, similar to order_by

I also added comma_seperated_arguments function that extracted out the
argument parsing so that it can be shared by both group_by and order_by
I am not sure why you had such a long parse function when it can be
handled simply with "join(',')".
added group_by
Merge pull request #3 from serkanserttop/master

GROUP BY implementation (sorry for taking such a long time to merge in!)