Renzo/segment io refactor

@rocha and @shnayder, please review?

I've added a forced increment to the page load counter each time a user navigates to a sequential, creating the illusion of page loads. Each of these `pageview` events is associated with the ID of the sequential it was fired by. In light of conversations with Mark and Victor, I've also refactored my previous integration of Segment.io. We now call the API directly (instead of through the Logger), and the events have proper names and well-named fields.

In addition, I've added the ability to track which tab/page an instructor is on in the current instructor dashboard, as requested by Mark and Victor. This wasn't possible before since the URL doesn't change. Miles' refactor will require a forced "page load" count using Segment.io's `pageview` method, but this will allow us to track instructor navigation behavior before then.