Add v3.1.0beta to downloads (and update CDN links)
Fix page build errors since GitHub moved to a newer version of Maruku

GitHub's current version of Maruku won't build the site because it
performs strict XHTML validation and this (a) disallows use of inline
<style> tags and (b) wrecks inline <script> tags by adding
<![CDATA[…]]> wrappers that trigger errors in browsers. Fortunately the
alternative Kramdown processor appears to work perfectly.
Merge branch 'gh-pages' into gh-pages-3.1.0
Add docs for ko.computedContext