Added flip methods for vertical and horizontal mirroring

I just needed this functionality... so I've implemented it according to your code style and now I'm contributing it for the original repo.

Thanks and Cheers!

edit: btw.. I've tested this code in my PHP 5.3 server, so If you have a PHP 5.5 server with the imageflip function enabled please, give it a try!
Can you please add braces here?