Added late static binding support

Hi William,

Thanks for porting this to PHP! We've just started using your little library, but the results so far look promising. I've prepared a small pull request which replaces all `self::` calls with `static::`.

I did this so I could subclass the `EmailParser` class and add my own `SIG_REGEX` const, which I've extended with some localized signature strings (f.e. 'Met vriendelijke groet' (with kind regards), 'Verstuurd vanaf mijn iPhone' (send from my iPhone)).

Hope you will pull this in!

-b