Fixes #64. Updated validateDateFormat

Currently the below will validate as `true` which is incorrect due to the 27th month.
```
$v = new Validator(array('date' => '2013-27-01'));
$v->rule('dateFormat', 'date', 'Y-m-d');
```

`date_parse_from_format` which is used in `validateDateFormat` returns with an `error_count` of `0` and a `warning_count` of `1`. The warning is `The parsed date was invalid`.

I've updated `validateDateFormat` to check that both `error_count` and `warning_count` are equal to `0`.