Just install whatever's best for the OS.
Detect host os and architecture from the navigator properties. If we
can't make a good guess, then they get the tarball, and of course
everything we have is on the full download page.
- Improved styling of download links.
- index.html#download now redirects to /download/
- Added missing hyphens, and added the missing "and 64-bit" for the Mac
Installer.