Rearrange downloads page with 0.2-rc1 on top

We'll add in a "latest" or "0.3-pre" after 0.2-final lands.

@ihnorton I didn't change around your URLs, as I thought you might eventually make a new .exe off of the v0.2.0-rc1 tag proper.

Additionally, I will make a new ubuntu PPA for stable Julia releases and use that instead of the julianightlies in the Ubuntu section.  This shouldn't take too long, and after I do that we should maybe just remove the section about julia being included in Ubuntu 13.04, as that's v0.1.2.