Design: enhance the "download" button

[hugoroy]

• make alternative download links and alternative browser name addon/extension depending on browser used; (for instance: add-on for Mozilla Firefox ; extension for Google Chrome, otherwise just use the current link to downloads.html
• add a cropped/zoomed-screenshot according to the web browser, somewhere near the download button so that people understand what the extension does. I guess for this we could move the download button left.

Other improvements welcome!

[JustinBack]

Now In beta at tosdr/Frontpage#44