Add "active visitors" system to events.

This aims to help two goals:

  * Fixes #13 by providing a counter which hopefully makes the
    liveupdate page feel less lonely since you'll see how many others
    are also viewing it.

  * For the future: Provide a good activity metric which can be used to
    sort events when building a listing of active events.

When automatic polling is in place, the "pixel" should be fetched
periodically so that the number indicates people who're just watching
the page, not just refreshing, as well.