File caching & better compression for css

This can be merged immediately but I would like to explain what it does:

- build files moved from /public/build to /build
- minify all css, even vendor ones that weren't minified before

But the big one is:

- all files in /build are renamed with a md5 hash so that when they change the filename change

**This allows us to set caching for all files in /build!** I've set it to one year but it could be set to values even more far away in the future.

It should give a nice boost to loading times.

As you can see in static/layout.jade & index.jade I've added a small helper to make everythig automatic. File rename is handled by grunt.

Actually other than css & js files also the favicon is copied to /build (using grunt-contrib-copy). And like it everything can be copied except images referenced in css files.