Adds support for extension auto-updating.

This is a total crap pull-request, because I'm not sure how to test
this, so this is my best stab after following the [documentation
](http://developer.chrome.com/extensions/autoupdate).

Auto-update requires a bump in the version number, and that version
number must appear in both `manifest.json` AND the new file, `updates.xml`.

(I bumped it to version 1.3.)

Maybe at some point I'll look into trying to figure out how to compile
an extension so I can test it ;)