Adding integration tests for Digest authentication

This intends to prepare Digest implementation, to prevent regressing from the current (cURL) implementation.

Note that this requires an npm install http-auth; I'm quite uneasy on how to require it: what is the usage regarding this? Should it be included in the composer, or will it be sufficient to mention this new requirement, along Node.js, on the blog?
(the RingPHP pull request https://github.com/guzzle/RingPHP/pull/14, to include Digest auth in server.js, is not dependent on this nom install: the http-auth won't be required unless the specific Digest-requiring URI is called)