Move from MIT license for the blog

MIT is a good license for code, but not so much for the kind of text of the blog. Chose CC Share Alike since I think we should allow people to even make commercial use of the posts, as long as they attribute:

http://creativecommons.org/licenses/by-sa/4.0/

This will also need to show up in the generated HTML, I've looked into it but want to check with @alexfish before going ahead.