Allow alternative joining with WHERE IN for multiple plural joins

[jschreuder]

Adding multiple hasmany/many-many objects has the problem that it will multiply your output. Thus adding a relation with 3 objects to a single parent will return 1 x 3 objects. But adding another relation to your query with 5 objects makes it 1 x 3 x 5 = 15 rows for 9 objects. And it get progressively worse after that.

To solve this subsequent to-many relations could be fetching using an extra query:
SELECT .... FROM relation WHERE id IN (parent_id_1, parent_id_2, etc...)

What has to be figured out is how to configure this (seperate method from relation() or a flag in relation method second param array). The current get() method with the relation classes could probably be modified to work with plural objects as well.

It'll probably be a method named include() that includes a relation with the object without joining, while relation() will join a relation on the parent object. If someone has a better name, let me know...