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/74619/247227/c1d77514-8acd-11e2-8665-cfda23ceef4c.png)