Better download link

The current link redirects to the wiki anyways, this new link anchors
directly to the download section.
Remove empty inline CSS
CSS formatting

Changed to a more conventional format
HTML formatting

Add missing head tag, adjust indents
Add voluntarist logo

Add voluntarist.png, put it in the header, and style it
Merge pull request #1 from thofmann/master

Various small changes by @thofmann. Thanks!