changes in create function

Hi

I'm using Stringy and need to extend class, but for now static function 'create' uses 'new self' and i can't extend Stringy without copy create function to child class. For this I've changed to 'new static'.

As i see in composer.json min php version is 5.3 so all will be fine.
http://php.net/manual/en/language.oop5.late-static-bindings.php