install Flashlight Cert Installer Firefox Extension instead of Lantern Proxy Configurator

Still TODO:

- ensure this works on all supported platforms (so far tested with OS X 10.9.2 + Firefox 28.0)
- look into the add-on installation warning pictured below to see if it's avoidable (to start, try removing the Lantern Proxy Configurator Extension, have Lantern 1.3.1 re-add it, and see if Firefox gives the same warning for that)

![screen shot 2014-05-15 at 13 35 41](https://cloud.githubusercontent.com/assets/475147/2989831/308341b2-dc69-11e3-8055-261c93e15bb8.png)

Wanted to open a PR early though since the above shouldn't block review.