Reorganize Model::$belongs_to, Model::$has_one

[shmax]

Reorganizing Model::$belongs_to and Model::$has_one from this:

 public static $has_many = [
        ['payments'],
        ['people',
            'through'    => 'payments',
            'select'     => 'people.*, payments.amount',
            'conditions' => 'payments.amount < 200']];

to this:

public static array $has_many = [
        'payments' => true,
        'people' => [
            'through'    => 'payments',
            'select'     => 'people.*, payments.amount',
            'conditions' => 'payments.amount < 200'
        ]
    ];

This makes static checking on the options possible, and easier to read IMO.

TODO: a lot of the tests rely on static members like Model::$belongs_to, Model::$has_one, and so on. Some of the tests would try to cache and restore the state of these, others would append, still others would reset, which can lead to chaos. I think what we need to do is figure out how to do some kind of before_test sort of thing where we always blank those out, or maybe do some kind of mocking, or something.