Honor primary_key option in $belongs_to

[tuupola]

Fixes #364 primary_key support when using belongs_to relationship. With the following code:

class Car extends ActiveRecord\Model {
    static $belongs_to = array(
        array("alarm", "foreign_key" => "slug", "primary_key" => "slug")
    );
}

var_dump(Car::find($id)->alarm);

generated query before the patch is:

SELECT * FROM `alarms` WHERE `id`='something' LIMIT 0,1

after the patch it will be:

SELECT * FROM `alarms` WHERE `slug`='something' LIMIT 0,1

[jpfuentes2]

Also great and similar to #368. Do you think you could add a quick test to this?

[tuupola]

I actually looked at writing the tests. However I had hard time trying to understand the models and how to write the actual tests. I do know how to use PHPunit though :)