made some visual edits

made some visual edits:
- added ```github_repo``` variable in ```_config.yml``` which is used for the footer's github page link instead of going to the tldr user's page.
- added ```rel="author"``` attribute to the three authors, maybe google indexing would improve by it.
- meta tag ```content="noarchive"``` is used to prevent google from showing old cached versions, because who the heck will look at those?
- applied some consistency by bringing the same letter spacing format to the footer's tldrcode