Add sha256 prefix to index keys for artifact hashes (#290)

* Add sha256 prefix to index keys for artifact hashes

This change adds the `sha256:` prefix to index values that are created to simplify searching the transparency log for artifacts. In case we shift to using a different hashing algorithm in the future, this will provide a way to specify it.

Fixes #289

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>