Dump the oplog.
Add oplog-focused tests.
Fix test oplog marshalling.
Strip oplog entries for ignored databases.
Unmarshal oplog.bson all at once (a simpler approach).
Switch to removing ignored databases upon restore (excluding "backups").
Make the non-ignored databases more obvious in each test.
Make it clear why we verify that an ignored database was *not* stripped.
Merge pull request #1040 from ericsnowcurrently/backups-oplog

Handle the oplog when dumping mongo.

We are going with option 3 on https://lists.ubuntu.com/archives/juju-dev/2014-October/003818.html, with the caveat that the backups DB will be left out of the dump.