Support for date time objects

Not sure about this PR -- but hopefully this at least raises the issue.

The following doesn't work at the moment.

````
$qb->where($qb->expr()->lt('date', $post->getDate()));
````

This PR converts the date time object into a string, formatted like `2013-05-10T00:00:00+01:00` as I don't think there is a native way to do this with PHPCR? I seem to recall a discussion on the cmf mailing list about date times.

As I say, not 100% on this, so comments welcome :)