CLONE -Limit and offset in QueryBuilder

Currently (2.0 Alpha 1) QueryBuilder does not support limit or offset functionality. These were supported in Doctrine 1 and should be added. QueryBuilder's value is significantly lower without these features because simple pagination is not possible. Methods limit($limit) and offset($offset) should be added to QueryBuilder, like in Doctrine 1. Query already has support for limit and offset, so this should not be a large change.