Add uglify-save-license

Cureently, the optimized JS file doesn't include any license comments, though the original JS file is licensed under [the Apache License 2.0](https://github.com/google/web-starter-kit/blob/f4b1aa9e5a42ace6085293f138a62987644f2fc7/app/scripts/main.js#L1-L18).

To fix this, I added [uglify-save-license](https://github.com/shinnn/uglify-save-license) to this project.
It can preserve only license comments, even if the license comment has no [*preservation* directives](https://github.com/mishoo/UglifyJS2#keeping-copyright-notices-or-other-comments) such as `@license`, `@preserve` and `/*!`.