Add back Rakefile and full _config.yml; remove _site from repo

 - the _site/ directory is just for locally viewing the jekyll-built
   site; you don't need to push it to github

 - I think you want that Rakefile; I personalized the relevant bits
Merge pull request #1 from kbroman/gh-pages

Add back Rakefile and full _config.yml; remove _site from repo