add ability to create simple relations with addRelation()

[romaninsh]

Currently using hasOne() and hasMany() you can define related models. That's cool, but sometimes you want to add a custom relation.

Basically when calling $m->ref('AuditLog'); it would respond with a related model. Technically you could create a method $m->getAuditLog(); but we want to keep all relations separate in case we want to iterate through them (for example by visualising in UI or exploring relations).

Here is invocation:

$m->addRelation('AuditLog', function($m) {
    return (new Model_Audit($big_data_db))->addCondition('user_id', $m->id);
});

And usage after:

$log = $m->load(123)->ref('AuditLog');
$log->addAction(..);

or in UI framework:

$crud->addRef('AuditLog');

[DarkSide666]

👍

[romaninsh]

Partially implemented when introduced new Relation class.