New icon

r? @zalun @Steckelfisch

Adds default icon. Depends on changes in [new-icon branch in cordova-cli](https://github.com/mozilla-cordova/cordova-cli/tree/new-icon). I noticed the `create.js` was overwriting the `www` folder, which was later overwritten again by `prepare` with the root `www` folder. We should remove the entire `www` from this repo and use `platform_www` for Firefox OS specific files.