Update File.php

Added "offset" option to getThumb()
Update Resizer.php

Added "offset" options to resize() and crop()
Update Resizer.php

Added ```@param array $offset The offset of the crop = [ left, top ]``` to both the resize() and crop()
Merge pull request #18 from DanielHitchen/master

[Feature Request] Added "offset" to getThumb() options