Use Kramdown to interprete Markdown in Jekyll.

Add Gemfile for Jekyll and Kramdown as well.

Maruku, the default for Jekyll, shut down, so let's specify it:
https://help.github.com/articles/migrating-your-pages-site-from-maruku
Merge pull request #24 from burlingtonruby/kramdown

Use Kramdown to interprete Markdown in Jekyll.