Updated requirements.txt and .gitignore

The original PIL obtained via `pip install PIL` has not been updated since 2009. `pillow` is the PIL fork that has been updated to this day. The main website is [here](http://python-pillow.github.io/). The necessary documentation of the Image module is [here](http://pillow.readthedocs.org/en/latest/reference/Image.html) and the places where you used the Image module should still work. Also updated .gitignore.