Do minification with Grunt + Uglify.

Fixes #305, hopefully. And obviates the dependency on Google Closure, which is great.

@kriskowal wanted to run this by you first since it's a new part of the process that we'll both have to deal with. It's pretty straightforward: `npm install` as usual (to get the new dev dependencies), then `npm run minify`, which will nicely output the Gzipped size as well. But yeah, let me know if anything looks strange or in need of documentation.