Remove the unneeded selectivizr.min.js.
Move JS files in _includes.
Move normalize.css in _includes.
Update respond.js to v1.4.2.
Move Google Analytics to a separate include.
Simplify the url variable and use it everywhere.
Move source files in source folder.
Be consistent with headers markdown.
Improve sitemap.xml.
Remove html5shiv.js and respond.min.js and use the ones from OSSCDN.
Convert 404.html to md.
Update robots.txt.
Update _config.yml.

Remove unused variable and rename `current_version` to `version`.
Merge pull request #12 from XhmikosR/master

Minor improvements