Makes ImageGenerator consider file storage
Python 2.6 compatibility
Docstring for get_placeholder_image function
Python 3 compatibility - StringIO replaced with BytesIO
Tests are finaly passes
Merge pull request #41 from AndrewPashkin/master

Makes ImageGenerator consider file storage
Only import placeholder module if needed. This prevents PIL imports when we don't really need it.
Removing Pillow from install_requires. It's not really required. Only if you want to use the ImageGenerator.