add support for new ObjectId(Date|Number)

see https://github.com/marcello3d/node-buffalo/pull/11, please.
Then could u check the following shell scripts:
```javascript
var timespan = Date.now() - 10*60*60*1000;
db.myColl.find({_id: {'$gt': timespan}}).count();
```
I would get the latest 10 hours records from their insertion date, It's useful.