Sass/scss port of styles

Re: https://github.com/videojs/video.js/issues/1376

I've added a `scss` port of the `less` file, which also has 3 additional variables for the `@font-face` declaration so it can customised. Mixins have been moved towards to top (they need to be declared first in sass) and documentation has been amended where appropriate.

I'm not familiar with grunt (I'm a gulp kind of guy), but I've added `grunt-contrib-sass` to the `package.json` file.

Wherever there's a task to copy, compile or minify the `less` and `css` files, I've added a twin task to do the same for the `scss` files and saved the files with the suffix of `-scss`, e.g.

```
video-js-scss.css
video-js-scss.min.css
```

I've also added an additional directory onto the `clean` task, which removes the `.sass-cache/` directory generated by sass compilation.

Sorry if any of this is wrong or I've gone about it in the wrong way. This is my first pull request.