Query Google Analytics for file usage information and display with Flot

This refactors some code originally written by @mjgiarlo to display pageview statistics in graphic form, and incorporates download statistics as well.  We use Google Analytics to gather the data and then query it for display within a special stats view.

A refactor was required because it made more sense to combine the two different metrics (pageviews and downloads) into their own model, which would allow for additional metrics to be added later or overridden by the user, as well as migrations to a different statistical gathering methods if you don't want to use Google Analytics.

It also addresses issue #403 with additional instructions for setting up Google Analytics to track your website.