Added image_converter how-to

Made a how-to detailing how to convert graphlab images to PIL images,
rotate them, and convert them back. Put this in an apply function.
Split up to_pil and from_pil into 2 how-tos
Addressed Zach's comments on how-to's
Removed titles
Merge pull request #11 from graphlab-code/image_converter

Split image_convert into to_pil_image and from_pil_image