Start to add support for hierarchical facetting (pivot).
Finish support for facet.pivot queries.

- fix option output from list to concatenated string
- new query command 'pivot_by'
- option 'mincount' is recognized for facet.pivot.mincount, rather f.FIELD.facet.pivot.mincount
- remove all other options
Add facet.pivot to output of facet_counts.
Fix broken test.
Update Changelog.
Merge branch 'master' of https://github.com/florianpilz/sunburnt into florianpilz-master

Conflicts:
	Changelog