Improve serialization of query class

Serializing the query class is currently possible by simply calling serialize($query), but the results aren't very satisfying as the index object is completely serialized, too. This, of course, leads to a massively larger serialized string than would be necessary. Therefore, modules like Saved Searches have to implement their own way of serializing query objects.

With an almost trivial implementation of __sleep() and __wakeup(), we can easily solve this.