Adds DevTools Extensions gallery

Adds:

* A DevTools Extensions gallery powered by a Google Docs spreadsheet and the Spreadsheet API. Items are positioned based on our own weighting factors in the sheet.
* Modelled after the CWS landing pages. This version will need to live in a framebox as we're using our own scripts.
* This version was originally using AngularJS and a number of directives, but I've rewritten it to only use jQuery and Underscore to trim down on page weight. When time allows, I'd like to refactor those out too. Template tag + XHR + qSA should give me most of what I need.
* Columns are super simple. Tried Flexbox but poor FF quirks + framebox requirement led me back to just using percentages. We can improve this in the next iteration. Flebox worked fine in Chrome otherwise.
* Our largest assets on the page are images, pulled from CWS image storage. Some of these are ~ the right size whilst others are original resolution. We may want to try either getting authors to fix these in CWS at some point, or more practically hosting smaller-res versions of the images ourselves locally.

This is how it looks in both FF and Chrome:

![screen shot 2013-12-11 at 13 21 10](https://f.cloud.github.com/assets/110953/1724103/b93965ca-6268-11e3-8166-e862392b458f.png)