Gruntfile.js: Use user-friendly properties.

BS3 commits: da8daa667cad6a872fd16c1e76338e2e7e3e2a9e
Gruntfile: switch cssmin & jade tasks from `files` over to `src` & `dest`

BS3 commits: 559f16572f6c1f0967ee7fcd72c07c67423d85d8