added basic support for firefox OS and firefox desktop apps WIP

hey :)

If you use Firefox Aurora or Nightly, this allows you to easily install devdocs cross-platform

e.g., on Mac, clicking the install will make a `devdocs.app` that you can easily launch.  Awesome!

Firefox for Android can also install this kind of app (and they will appear on the home screen like a native android app)

Let me know what you think, I have some questions within the code

TODO:
- [ ] add mimetype support for manifest to app.rb
- [ ] submit to firefox marketplace
- [ ] find out where to toggle the display of the `firefox-app-supported` class