Add base64_digest for subresource integrity

This PR adds `Asset#integrity` to simplify the implementation of [subresource integrity](http://www.w3.org/TR/SRI/#htmlscriptelement-1).

/cc @josh
Minor perf thing, I would like to avoid doing a full memory read of the file contents twice. We do it once for the regular digest but that also have some *smart* chunked reading stuff. Ideally we could generate one `Digest::*` class and just access the hexdigest and base64 parts.

Maybe for 3.x we could just kill `digest_class` configuration and force everyone into the SHA-256 future. Or allow non-sha256 right up until December 31, 2015 (kidding)

I'll look into this later.