Measure the benefit of compressing CSS

-- __Warning__ -- This pull request shouldn't be merged, it's just a conversation starter :smile: 

The current build lets SASS minify our CSS, however the algorithm used might not be the best.

I've added a grunt task, `css-benchmark`, that measures the compression ratio of our files using different algorithms.
The whole report is available in `tmp/css-benchmark.html`

What's interesting is that running `csso` on the already minified files we can go from `560305` to `525270` bytes (6% gain, `~34kB`) of the __gzip__ size.

Even better results could be achieved by running different compression algorithms to different files (`~34.5kB`), but that would be slightly more complex.
For instance `csswring` performs better for `webfonts` while `clean-css` is the best algorithm for `old-ie` files.