Syncing up

This pull request fixes a couple of bugs involved with saving and finding documents. The only API modification is that the `find()` method can now accept a list of IDs, which serves as a nice shorthand for `{_id: {$in: listOfIDs}}`.

I've also removed the warning section in the README. We've been using this in production at poptip.com for the past several months, so I'm pretty sure (read: really hope!) it's stable.