Make links/styles/images relative

This should allow the site to be deployed at any level in a directory hierarchy and it will still render/work correctly because the links/style/image refs are relative. In particular it should appear correctly and all links should work in the Jenkins workspace when the site gets built. I.E.

https://buildhive.cloudbees.com/job/jclouds/job/jclouds.github.com/ws/_site/index.html

The ultimate goal of this is to make doc contribution easier. If the site renders/works correctly in the Jenkins workspace, contributors can simply edit a Markdown file in GitHub, submit a PR, and check the result in the workspace. Anyone will be able to improve the jclouds documentation entirely from within their web browser.