Use Firefox extension installed dialog

Fixes https://github.com/diegocr/GitHubExtIns/issues/7

![Firefox extension installed dialog](https://f.cloud.github.com/assets/55841/2238662/8c7f6018-9bfa-11e3-9804-658d6ef94753.png)

This PR removes the pref `'nme'`.

Please review my code, as I couldn't test an addon failure.

Todo:
- [x] Use addon icon in dialog.
- [ ] Test for Android.