Updated downloads page and linux-install docs

Updated downloads page and linux-install docs to use the keyserver and to explicitly note that it's not Debian-only.
Hm, I think if people have to click one more time to get to the content I'm fine with that rather than exposing semi-complete info on this page.

Here's what I have in mind, do you think that'd be a good compromise (the links point to the respective sections in the install guide)?
![screen](https://cloud.githubusercontent.com/assets/1376924/4797256/4473f70c-5e0a-11e4-9472-9161ac82e8d7.png)