Add fingerprint to the saved meta data

I forked this gem to allow us to store the meta data about our offer images into the database.  It provided 99% of the framework to do that.  It was missing one core piece of information that we needed though, which is the md5 hash of the image.   This PR updates the forked gem to include the `fingerprint` (i.e. md5 hash) of the image into the meta data.

The next step will be plugging it into our offers.

PT story: https://www.pivotaltracker.com/story/show/76550702