add a basic Javascript UI

The Javascript is pretty amateurish I'm sure but at least it's something to
iterate from. It's already much more pleasant for browsing through videos in
several ways:

* more responsive to load only a day at a time rather than 90+ days
* much easier to see the same time segment on several cameras
* more pleasant to have the videos load as a popup rather than a link
  that blows away your position in an enormous list
* exposes the fancier .mp4 generation options: splitting at lengths
  other than the default, trimming to an arbitrary start and end time,
  including a subtitle track with timestamps.

There's a slight regression in functionality: I didn't match the former
top-level page which showed how much camera used of its disk allocation and
the total duration of video. This is exposed in the JSON API, so it shouldn't
be too hard to add back.

README.md

@@ -10,10 +10,12 @@ analyze, or re-encode video frames, so it requires little CPU. It handles six
 2](https://www.raspberrypi.org/products/raspberry-pi-2-model-b/), using
 less than 10% of the machine's total CPU.
 
-So far, the web interface is basic: just a table with links to one-hour
-segments of video. Although the backend supports generating `.mp4` files for
-arbitrary time ranges, you have to construct URLs by hand. There's also no
-support for motion detection, no authentication, and no config UI.
+So far, the web interface is basic: a filterable list of video segments,
+with support for trimming them to arbitrary time ranges. No scrub bar yet.
+There's also no support for motion detection, no authentication, and no config
+UI.
+
+[![screenshot](screenshot-smaller.png | 704x474)](screenshot.png)
 
 This is version 0.1, the initial release. Until version 1.0, there will be no
 compatibility guarantees: configuration and storage formats may change from