To support Pillow

Pillow is a fork of PIL which supports Python 3 and fixed some issues of PIL.
See details here: https://pypi.python.org/pypi/Pillow/2.0.0

I want to use PyCAPTCHA in my project but I am using Pillow 2.0.0 in my project instead of original PIL. So I changed a bit of codes in PyCAPTCHA to make it support both PIL and Pillow.