added noop function to Utils for comfier cb management
added support for syntax Document.save(cb, errorCb)

When errorCb is defined the signature of cb is also changed from
cb(err, doc) to cb(doc).

It's great for express:

app.all('...', function (req, res, next) {
  (new Doc({req.body})).save(function (document) {
    ... do something else ...
    res.send(document);
  }, next);
})
refactored insert and destroy

See commit 8f270b7517a48bbc3a45ffd3293abe9d2ac7a05e.
typo
added merge functionality