challenges.html update

- edited to 4 tabs in a row
- edit tab headings
Merge pull request #26 from ChrisBoesch/master

update
resloved digest issue, added a simple pop up example for Chris
Merge pull request #92 from LI-HE/master

resloved digest issue, added a simple pop up example for Chris
Merge pull request #33 from ChrisBoesch/master

Updating my repo