Update auth flow

This updates the authentication flow for GA to use environment variables rather than storing credentials in files.  This is good practice, but also needed so we can easily fetch analytics from a jenkins job, as we want to for https://www.pivotaltracker.com/story/show/70737424 (updating page traffic information in search nightly).

It also tidies up the GA response caching code; in particular, it makes old cache entries expire so we don't run out of space eventually!