Added a function to validate hex-encoded MongoDB ObjectId.

MongoDB's ObjectId are widely used with Node.js. They're represented in hex format as 24-characters strings.
This adds the isMongoId() function to validate them.

PS: @chriso I know you must be starting to hate me for all the PR's :)
Merge pull request #324 from EgoAleSum/patch-mongo

Added a function to validate hex-encoded MongoDB ObjectId