Switch to grunt-usemin.

TODO:

- [x] Fix the issue with pack.css and pack.js not getting minified, just concatenated
- [x] Fix html5shiv being zero byte
- [x] Re-enable unccs
- [x] Remove fonts from rev since they include the version in their filename
- [x] Add images to the rev task
- [x] Add back the dev target which does not minify files etc