Change the absolute path to {{ site.baseurl }} to make hyperlinks and resources valid when published as a project page.

When published as a project page, since the base url is username.github.io/projectname/, most links in the site break. Using site.baseurl could enable correct links and resources path both on local machine and on github. 

When pushed to gh-pages, one should add `baseurl: username.github.io/projectname/` to _config.yml. **The trailing / is essential**

I left id part in the atom.xml to be site.url.