Updated tobytes to tostring method as per latest PIL available on effbot...

Hey,

I just installed latest PIL using easy_install and while calling ::

    img = Image.open("test.png")


I was getting an AttributeError for tobyte() which seems to have been removed from `PIL Image module: https://bitbucket.org/effbot/pil-117/src/f356a1f64271e9d3206fcf9059492ba1c9e163d6/PIL/Image.py?at=default