Merge pull request #157 from atlasapi/publisher-content-query

Add query by publisher for content
Add content purger

Sometimes it's necessary to remove all content for a
publisher, rather than just mark as !activelyPublished.
The ContentPurger does that, with the added bonus of
being able to retain equivalences on the Content that
remains, so they can be restored if the purged content
is re-ingested.
Merge branch 'purge-content' into stage