Refresh download page

The download page is looking rather dated and is difficult to navigate.  I'm not a graphic designer but I thought I'd try my hand at making it look a bit more modern and easy to use.

You can see what the [full page looks like here](http://dlang.gnuk.net/download.html).

![new-downloads](https://f.cloud.github.com/assets/74619/247258/329bb90c-8ad0-11e2-95cc-aa164ea4f292.png)