Update for Bootstrap v.3

Thanks for making this quiz public and open. I use it in my classes at UNC jSchool and many of my students integrate it into their projects. 

I have updated the files to reflect the new boostrap 3 class names. Mainly changing span12 to col-12. I have done basic testing locally but happy to make changes if you see something I missed.

Respectfully requesting a merge to upgrade to bs3. You can reject my README.md changes as it simply notes the difference in the demos which if you merge would not differ.

Thanks,
Steven