Add testing for CR2 files from other cameras.

I've added a bunch of tests images for other Canon cameras that use CR2 as found on http://www.rawsamples.ch/.  I've also hosted a copy of all the images in a shared folder on my Google drive (which I'd be more than willing to give you edit permissions to).  This change modifies the tests to prefer fetching from Google over the original hosts and also verifies the checksums before running tests.

I have some thoughts on how to make thumbnail generation faster in camlistore, but I wanted to beef up testing before i started going down that path.

I'm a Googler with the same username as on github if you need a CLA.