Attach footer to bottom

I think I've correctly translated the CSS modifications from https://github.com/sniksnp/gatling.github.io/commit/e68e4216e1fc3cb790d63c3087006e615b2ef2e9 to LESS, but I haven't compiled the site yet (need to install npm, grunt, etc.)

Could you check if it works on your side before merging, please ?