Adding current user available to pre/post save hooks

This PR makes the current user available to `pre` and `post` save hooks.

I do not alter the model per se. I simply add a `_user` property to the `item` object just before it is saved in `updateHandler`, but since `mongoose` is unaware of it's existence, it does not save it to the database. The property is temporary in nature and does not survive the current transaction.

Here's a usage example.

	List.schema.pre('save', function(next, done) {
		console.log('current user =>', this._user);
		next();
	});

The idea of having the current user available to `pre` and `post` save hooks was originally raised in #458 in the context of using it for audit purposes (which I addressed in #490). However, having the current user available in `pre` and `post` save hooks is also quite useful in use cases other than this, which is why I decided to submit this as a separate pull request.