Add watermark support

Credits:
- Vadym Zakovinko <vp.zakovinko@gmail.com>
- Trurl McByte <trurl@mcbyte.net> (http://forum.nginx.org/read.php?21,235958)
Add center option to watermark_position

Credits:
- Rene Diepstraten <mail@renediepstraten.nl>