Support for stronger hashes as alternatives to MD5

Presently Digest authentication is based on MD5 - however we should either update the mechanism or add new mechanisms to support the use of stronger hashes.

As this library is used both client and server side installations that require the stronger hashes can just ensure the client and server have the latest version of this library - installations that still require interaction with MD5 will need to ensure that it is still available as a mechanism.