Add support for strtotime format in (s)maxage

With this PR I would like to enable support for strtotime format in maxage and smaxage property of `@Cache` annotation.

If merged, it would be possible to write : 

```php
    /**
     * @Cache(smaxage="2 days")
     */
    public function getCountryAction(Country $country)
```

The format of the duration is the one supported by the parser of strtotime()

http://www.php.net/manual/en/datetime.formats.relative.php

ATM tests don't pass because of an unrelated issue.