change doctype from html to xhtml

Necessary since github updated to use a newer version of maruku, which wraps javascript in a CDATA section. This breaks if doctype is html (see https://github.com/bhollis/maruku/issues/120).
Merge pull request #8 from jrrideout/pub-list-fix

let's go
update to more closely match github pages setup

now uses kramdown, etc.