Code review comment for lp:~leonardr/lazr.restfulclient/basic-auth

This branch changes the way lazr.restfulclient does (or doesn't do) authentication. Previously, the ServiceRoot constructor took a "credentials" argument that didn't do anything, but that could be used by subclasses. This branch introduces the idea of "authorizers", objects that are used at specific points in the lifecycle to make sure HTTP requests are made with the correct Authorization header.

This change makes it easy to add code for common authorization techniques to lazr.restfulclient. The only authorizers in this branch are one that doesn't do anything (only useful as a superclass) and one that supports HTTP Basic Auth. I'll be moving in the OAuth authorization code from launchpadlib as an authorizer.