PROTOTYPE: port to firefox devtools addon
fix styles in firefox
ignore generated xpi
choose an 16x16 icon as Firefox DevTools panel icon
minor firefox css tweaks
added ff_extension_dist/data/vendor and ff_extension_dist/data/images to .gitignore
integrate Mozilla Addon SDK download and xpi generation

- add grunt-mozilla-addon-sdk to npm devDependencies
- configure new grunt tasks "mozilla-addon-sdk:download" and
  "mozilla-addon-sdk:xpi"
register build_xpi grunt task
FIX: configuring grunt-mozilla-addon-sdk to download 'firefox26' revision

needed to get a Mozilla Addon SDK version which supports chrome.manifest (http://bugzil.la/559306)