TransferBehavior: Add support for simple rules and one-rule-per-field validation

This patch adds support for using simple rules:

```php
public $validate = array(
	'file' => 'ruleName'
);
```
as well as the one-rule-per-field style:

```php
public $validate = array(
	'file' => array(
		'rule' => 'ruleName'
	)
);
```

while keeping the "merge with default validation options" behavior.