Add some new metadata fields to flambe.yaml.

description and developer. These will be used more later.
Add experimental Firefox OS target.

Documentation pending, but the gist of it is:

- Ensure your flambe.yaml has the new fields.
- Your project should have a 128x128 icon.
- Run `flambe build firefox`.
- Load build/firefox onto your phone/simulator (read this:
https://developer.mozilla.org/en-US/Firefox_OS/Using_the_App_Manager)