Upgrade to universal analytics.

The universal analytics are now available as a public beta, and they're
significantly better suited to what we're trying to do. For starters, there's
actual documentation and we don't need to use reverse engineered fake cookies.

In addition to our existing event tracking, it's also easy to track screen names
and limited timing information now. We can refine what exactly we track, but
this should be interesting.

Note to concerned readers: You can still opt out, and it's still true that we're
not tracking what exactly you watch or where it came from. Screen tracking will
help us track workflows and make future improvements.