Create collections if they aren't there with save()

I don't know if there's a better way to do this, but I added a function to automatically create collections if they aren't there.
In regular mongo, it'll save data to a collection without having to explicitly create it first, so having this makes using it easier ;)

I did it as an exception, to avoid having to do a sql lookup before each insert.
I've never written plv8 before, but it *seems to* work ;)

-CPD