add isValid validator method to ObjectID object

The logic to validate an ObjectID was already here so I figure rather than creating my own validation object, it would make sense to expose a method to check the ObjectId is ok :)
Others are looking for this functionality on an ObjectID (e.g. http://stackoverflow.com/questions/11985228/mongodb-node-check-if-objectid-is-valid).
My first ever pull-request! :)