feat(release_util): Add support for signing individual file uploads.

The .DIGESTS format is clunky and annoying. It also requires uses to
perform two steps to verify images using GPG. Instead support signing
all files directly so there is no need for .DIGESTS.

The old DIGESTS code will remain in place for now but after a few
releases I plan on deleting it.
Merge pull request #230 from marineam/gpg

feat(release_util): Add support for signing individual file uploads.