Installer download tracking.

Tested in Chrome and Firefox.

The beef is in 

https://github.com/getlantern/lantern_aws/blob/0089d7295f2e159d9bfa961202b35b6cf8cb23d2/salt/fallback_proxy/installer_landing.html

These landing pages are parameterized with the installer URL and platform (for reporting).

To deploy this we would just call bin/update.py and then run the script to rebuild installers (rebuilding/uploading installers isn't strictly necessary, but it's quicker than doing a script that renames the existing installers prior to uploading the landing pages).