Signature method extensions

Due to popular demand, it has been suggested that we assemble a list of signature method names and a standard definition to enhance interoperability and consistency of libraries. It is not realistic to expect libraries to only support PLAINTEXT, HMAC-SHA1, and whatever RSA we end up adding.

We agreed to drop MD5, CRC32, and the likes from the spec due to security concerns. However, those signing algorithms should still be documented and will be supported by vendors so we might as well provide a consistent way of using them.