Refactor the model initialization step & generalize attributes inheritance on non array.

- All autoConfig related attributes don't inherit form parents by default.
- `Model::_inherit` allow a custom attribute to inherits form parents

```php
class MyModel extends \lithium\data\Model {
	protected $_custom = array( // will inherits `array('prop1' => 'value1')` form it's parent
		'prop2' => 'value2'
	);
}

class MySubModel extends MyModel {

	protected $_inherit = array('_custom');

	protected $_custom = array(
		'prop1' => 'value1'
	);
}
```

The following attributes are inherited by default:
`'validates', 'belongsTo', 'hasMany', 'hasOne', '_meta', '_finders', '_query', '_schema', '_classes' `
			'_initializers'