Support for NULL fields

Sometime I need to send NULL values into $items array to database, and currently it gives me:

Catchable fatal error: Argument 1 passed to Cactus\Reverter::dateTime() must be an instance of DateTime, boolean given

Just checking there is an actual value into $value will solve it
Merge pull request #2 from lordoffreaks/patch-2

Support for NULL fields in Reverter Class