Switch fully to Webpack

[ttencate]

The Grunt + Webpack approach doesn't allow for an efficient development workflow; we can only run the entire pipeline again if something changes. But I think we can drive everything from Webpack and get some extra goodies as a bonus (ES2015, SCSS).

• Replace grunt-contrib-jshint by eslint-loader
• Replace grunt-contrib-copy by file-loader
• Replace grunt-contrib-cssmin by sass-loader
• Replace grunt-contrib-imagemin by image-webpack-loader
• Add support for ES2015 using Babel, compile it to ES5 or even ES4 [pointless, because AudioContext requires a fairly modern browser anyway, and pretty much implies ES5 support]