* Fix absolute URL generation
url and baseurl config for this theme deviates from jekyll standard,
and that results in double inclusion of project path
if this theme is used as project page.
Switching to Jekyll accepted url usage so that
absolute_url filter works.
* Alternate links need to be aboslute
* mention url and baseurl only need to be modified in local dev
* Update readme on new url/baseurl setting changes
* Fix showing background img with non-empty baseurl
Also switch globally to relative_url for all baseurl prepends,
which is more robust than custom URL mangling
* More site.baseurl -> relative_url conversion
