Re-add a search dropdown to the beta site.

This uses Twitter's typeahead.js for the actual autocompletion, and uses our
existing search-index.json and search-description.json files. These are pretty
big, and an obvious fix down the track would be to combine them into one,
preferably much smaller file. mod_gzip should help to some extent, as should
the HTML 5 Local Storage caching I've implemented.

I've also pulled in Hogan.js for templating, since typeahead.js really needs
some sort of templating engine to be useful.

Excluding the JSON files, the added JS dependencies are around 50k
uncompressed. I think this is reasonable, but honestly, it's not really a
significant win over the autocomplete bits of jQuery UI. I've used typeahead.js
more for its multiple section functionality than anything else.

The styling on this is pretty average, but I'm mostly interested in testing
functionality right now. Fixes welcome.
Various styling updates, including responsiveness (gasp!).
Darken the general search option.