Redesign layout for wider pages, add more links

... to the main page.
Add line break after download button
Add workaround for HTML5 semantic elements in IE 8
Change 'comment on reddit' to 'comments on reddit'

Posts for older entries might already be read-only.
Add big OS icons next to download button
Make sidebar links slightly brighter
Compact icons map
Add title to OS download icons
Make small OS icons monochrome, use Stycil Tux
Add forum links to the contact page
README.md: Add requirements and build instructions
Fix HTML errors
Also turn of iframe border for YouTube in CSS

The frameborder attribute is obsolete in HTML 5. Keep the attribute
though for backwards compatibility.
Implement sepia filter for non-Firefox browsers

Also add grayscale fallback for IE.
Use a 'less bold' icon for FreeBSD
Correctly name link to the Steam Users' Forum