supported Pillow and fixed README styles

I was trying out Shellpic via `pip` and got error about no PIL, then realized my system has [Pillow][], which is a fork of old PIL. So, I cloned the repository, but got an import error about `Image` module can't be found.

[Pillow]: https://pypi.python.org/pypi/Pillow

I made a quick file by using `from PIL` import, then I thought I could make the old PIL dependency more flexible with Pillow, hence this pull request.

I tested on Python 2.7.5 with Pillow 2.0.0. Please take look at the comments in `setup.py`, Pillow version isn't checked. The interface might be the same from 1.0 to 2.3.0 (latest), or just Shellpic doesn't use something has been changed.