Ataki/gridfs

@stvstnfrd to review commit
At a glance, it looks like this could break for any invocations using
the former method signature which didn't require the options parameter.
I don't know where else this is used, so does it make sense to preserve
backward-compatibility and assume `options` could be
empty/null/undefined and assign it a default value?
```javascript
function (options) {
    options = options || {};
}
```