overhaul download page

- replace tables with a list of divs
- link to ldc/gdc/VisualD release pages
  (instead of including outdated download links)
- remove any non-D related links
- use smaller/better OS logos

logo sources:
- http://commons.wikimedia.org/wiki/File:Apple_logo_black.svg
- http://commons.wikimedia.org/wiki/File:Windows_logo_-_2002%E2%80%932012_(Multicolored).svg
- http://design.ubuntu.com/wp-content/uploads/logo-ubuntu_cof-orange-hex.png
- https://www.debian.org/logos/openlogo-nd.svg
- http://fedoraproject.org/wiki/File:Fedora_infinity.png
- https://www.freebsd.org/logo/logo-full.png
- http://gitorious.org/opensuse/art/blobs/master/00assets/logo/official-logo-geeko-color.svg

processing:
convert {}.svg -transparent white {}.png
convert images/freebsd_logo.png -crop 175x175+0+0 images/freebsd_logo.png
mogrify -depth 16 -filter lanczos -thumbnail 1000x40 -depth 8 images/*_logo.png
pngcrush images/logo_*.png