Implement the archive delete API endpoint.

Also fixed tests not properly skipped in the case Mongo
has no JS support, and improved the test separation
when counters are exercised.
Update stats counters docstring.
Improve delete archive error tests.
Add a reference to the spec for archive delete endpoint. Add an additional test as per review.
Merge pull request #80 from frankban/delete-archive

Implement the archive delete API endpoint.

Also fixed tests not properly skipped in the case Mongo
has no JS support, and improved the test separation
when counters are exercised.