Remove autoload:ff from build:firefox task, and automate mozilla-addon-sdk installation.

- dreditor.org does not have Firefox, but we want it to build the extension, so autoload needs to be moved elsewhere (later).
- Automating the Mozilla SDK at grunt install time should help in on-boarding new contributors.
Merge pull request #126 from sun/grunt-build-install

Remove autoload:ff from build:firefox task, and automate mozilla-addon-sdk installation.