Add isCapped(callback) method to collection.js

I made a little bit alteration for your library, which will support `isCapped()` in that [official documents](http://docs.mongodb.org/manual/reference/method/db.collection.isCapped/#db.collection.isCapped) if u merge this request.
This change maybe match your 2nd TODO `Various utility methods`. So my question is solved, then looking forward your mergence.