Tidy up the README

After getting dartlang.org up and running locally, after using the helpful steps in the README, I thought I'd update the steps. This PR:

* changes most URLs to be links (saving horizontal space)
* nests lists less
* adds links for Windows installers
* Fixes the codeblocks inside lists (8-column rule)
* `Monospaces` directory names in text
* Uses a 100-column limit for text, but this is just my personal preference
Change "OneClick Installer" to be "RubyInstaller site"?

(I'm on a mac, so maybe I'm seeing something different, but I don't see the string "click", much less "OneClick", anywhere on this page.)