Code review comment for lp:~leonardr/lazr.restful/cache-service-root

UNfortunately, this branch breaks lazr.restfulclient due to what I believe is a bug in httplib2 (http://code.google.com/p/httplib2/issues/detail?id=97). We can still land this branch, but we'll only be able to serve the goodness to clients that signal they can work around the bug. This means changing clients to send custom values for User-Agent.