'group_by' is now a list

Backdrop needs to be able to handle being passed multiple 'group_by'
query-string parameters.
Nested merge now groups using a list of key combos

This means that when multiple group_by parameters are specified we can
group by all of them and include all of them in the same group of
results.
Merge pull request #339 from alphagov/multiple-group_by-params

Nested merge now groups using a list of key combos