Add lazy loaded relations

Perhaps this needs some refactoring to improve proxyfying...
Here is an example usage :

```yml
class: Entities\User
url: /users
fields:
    id:
        type: integer
    login:
        primaryKey: true
        type: string

    # ...

hasMany:
    organizations:
        remote: orgs
        targetEntity: organization
        lazy: true

```

```php
<?php
use Rizeway\OREM\Entity\Entity;

class User extends Entity
{
    public $id;
    public $login;
    
    // ...

    public $organizations = array();

    // ...
}

```

```php
<?php
$factory = new \Rizeway\OREM\Config\Factory('Mappings', 'https://api.github.com/');
$manager = $factory->getManager();

$user = $manager->getRepository('user')->find('jubianchi');
$userOrgas = $user->getOrganizations();
```