Base64 encoding should be renamed to base64raw

[ezekg]

Right now, base64 encoding via --checksum-encoding base64 and --signature-encoding base64 use Go's RawURLEncoding, which lacks padding. This doesn't play well with some tools, such as Rubygems. We should rename what is currently base64 to base64raw, keep it as the default, but also add base64 and base64url encoding options. This should be a backwards compatible change, since I doubt anybody is relying on an explicit base64 encoding, since it's already the default.