Add more auth options, including digest support

Add the capability to specify auth username and password in request
options or via Request.prototype.auth, as well as whether they should be
sent immediately as Basic authorization (set this to false when using
digest authentication).

Conflicts:
	main.js
Add tests for basic and digest auth
Document new auth options
Merge pull request #338 from nylen/digest-auth

Add more auth options, including digest support