Only show social profiles if there is a link

I want to have people share my stuff on facebook, but dont really like randoms adding me. Maybe thats rude, but facebook is a little more private than Twitter for example.
Merge pull request #16 from philsturgeon/master

Only show social profiles if there is a link in the _config.yml file. If their is no link the social profile link will be hidden on index.html. The sharing option in the article page remains untouched.