Add duration in seconds

Does what it says on the tin
In this cases, to avoid repetition and have tests that give more insight on what they're testing, I like to have helper functions such:
```php
private function _getSecsForDuration($text, $value) {
 $ver = $this->_createVersion(array(
    'duration' => (object) array(
        'text' => $text,
        'value' => $value
    )
));
 return $ver->getDurationInSecs();
}
```

Then your tests will just be
```$this->assertEquals(630, $this->_getSecsForDuration('11 mins', 'PT10M30S'));```

Which I think would read nicer