Use PIL library if provided.

Hi Jackpal,

I'm using Debian Linux. But I cannot find sips on Debian system.
However, Debian provides PIL library which depends on ImageMagick.
So I create a patch for picasawebuploader to use PIL when it exists.

Please review my patch and see if it is good to be upstreamed. And please ask me to fix the patch if it is not good enough.

Thank you very much,
PaulLiu