Propose _validation_date

I often implement the validation of date for 'datepicker' forms. 
So I propose that extension.
Can you check?

Example:
```php
		$post = array(
			'f1' => '2013/02/26',
			'f2' => '2013-02-26',
			'f3' => '2013/2/26 12:0:33',
			'f4' => '19 Jan 2038 03:14:07',
			'f5' => 'Sat Mar 10 17:16:18 MST 2001',
			'f6' => '',
			'f7' => 'foobar',
			'f8' => '2013/02/26',
			'f9' => '2013/02/29',
		);

		$val = Validation::forge('test');
		$val->add_field('f1', 'F1', 'date'); // OK
		$val->add_field('f2', 'F2', 'date[Y-m-d]'); // OK
		$val->add_field('f3', 'F3', 'date[Y/m/d H:i:s]'); // OK
		$val->add_field('f4', 'F4', 'date[d M Y H:i:s]'); // OK
		$val->add_field('f5', 'F5', 'date[D M d H:i:s T Y]'); // OK
		$val->add_field('f6', 'F6', 'date[Y/m/d]'); // OK
		$val->add_field('f7', 'F7', 'date'); // NG
		$val->add_field('f8', 'F8', 'date[Y-m-d]'); // NG
		$val->add_field('f9', 'F9', 'date'); // NG
		$val->add_field('f9', 'F9', 'date[,0]'); // OK
```