[Bug 878882] Generic way to do GA events on links.

Adding `data-ga-click="_trackEvent | Some thing | Another thing"` will cause `_gaq.push(['_trackEvent', 'Some thing', 'Another thing'])` to happen, and then 100ms later, the page will navigate as normal.

I used this to add `data-ga-click="_trackEvent | External Links | Top Banner - Firefox Upgrades"` to the "Your firefox is out of date" banner.

To test: Trigger the out of date Firefox warning. I use Fx 12 on Windows' user agent for this, then click the download link. If you watch net traffic, there should be a request to Google Analytics. Also do the same thing with the download button on the Firefox product page.

r?