Rework Main page

I have some comments around the main page.
Here is what I am proposing and willing to implement:

1. Display both Windows 32 and 64 in unknown and default instructions
2. Make unknown and default instructions way more compact so you don't need to have a huge screen to see the footer without scrolling (ideally all instructions would be the same height)
3. Get rid of the cycling feature which is flagged off at the moment
4. Add unix instructions also for wget (granted wget is suppported by the shell script)
5. Add Rust favicon
6. Add github "Fork me" ribbon instead of the "about rustup" link
7. Add a link to the Rust Community page instead of a link to Mibbit

Related to : https://github.com/rust-lang-nursery/rustup.rs/issues/1253 https://github.com/rust-lang-nursery/rustup.rs/issues/1369 https://github.com/rust-lang-nursery/rustup.rs/pull/1268

@nrc @withoutboats @alexcrichton @Diggsey Let me know what you agree / disagree with.

Start at the rustup.rs main page and review the seven proposed changes alongside related issues 1253 and 1369 and pull request 1268. Confirm which proposals remain wanted and identify the page assets or templates involved; the work is done when the agreed layout, instructions, links, favicon, and ribbon changes are implemented and checked in the browser.