This commit includes layout changes suggested by Christophe, and some CSS modifications to make the 'fork on github' ribbon scroll properly.

changed worksheet links to match updated filenames in io.

Updated links to codalab.github.io
Merge pull request #456 from willum070/landingpage

This commit includes layout changes suggested by Christophe
Merge branch 'refs/heads/master' into feature/phase-to-phase