Add multivariate test framework

(This supersedes #74 because GitHub messed up the PR.)

This is a client-side multivariate test framework for A/B testing stuff. It uses Google Analytics custom variables to segment users in different cohorts. See the comment at the top of multivariate-test.js for more info on how it works.

This has been initially built for this story:

https://www.pivotaltracker.com/s/projects/367813/stories/61474462

#74 was closed because we couldn't get GA to work with page-level custom variables. We now use session-level custom variables, which essentially allow us to filter users by whether they were in a particularly cohort at some point in their session. So we can, for example, answer questions like "what is the likelihood of a user that is on page X going to page Y if they were in cohort Z".