port to firefox devtools addon

This pull request is not intended to be merged as it is, I’m opening it to start evaluating if there's interest to integrate it into the project and, in that case to work on it until it will be ready to be merged (and then fix #59).

- demo screencast: http://youtu.be/qYCL4HOtddY
- experimental XPI build: http://ubik.cc/projects/ember-inspector.xpi

TODO LIST:
- [x] build firefox and chrome extension from the same sourcecode
- [x] firefox extension seems to work correctly, but there’re some CSS issues to work out.
- [x] integrate xpi build
- [x] test "grunt build_xpi" on OSX
- [x] support DOM inspection request
- [x]  how to integrate "images" linked from handlebars templates
        SOLUTION: using relative image urls (e.g. "../images/send.png")
- [x] merge port-firefox and port-chrome modules into port modules (app, ember_debug)
- [x] fix bug ember app not detected from file url or from localhost (NOTE: unable to reproduce)
- [x] remove findCssSelector
- [x] fix vertical arrow (app css)
- [x] fix main section vertical scrollbar (should be 100% and scrollbar disabled)
- [x] rebase to firefox-adapter
- [x] update README
- [x] integrate build_xpi into build_and_upload (upload last build xpi using ember-s3)