Implement magic method __isset to be compatible with Twig 

> "When Twig encounters a variable like article.title, it tries to find a title public property in the article object. It also works if the property does not exist but is rather defined dynamically thanks to the magic __get() method; you just need to also implement the __isset() magic method."
-- http://twig.sensiolabs.org/doc/recipes.html#using-dynamic-object-properties

As you might have guessed, this patch implements the `DomainObject::__isset` method. It also tweaks `DomainObject::load()` a little bit, as the implementation of the `__isset` method causes `load()` to be called with it's parameter (`$load`) being `NULL`.