Disable subsampling in JPEG images to improve the quality

By adding subsampling=0 to the PIL save method, the JPEG image quality can be improved drastically.

Here is an example (watch the red text in the image):

Original image: http://s12.postimg.org/5xajlw9al/cover_original.jpg
Image with subsampling: http://s11.postimg.org/wmi05mqw3/cover_with_subsampling.jpg
Image without subsampling: http://s12.postimg.org/saia8pa8d/cover_without_subsampling.jpg

It would be great if you could merge this pull request into the master. Thanks!