Update to Jekyll 2.2.0.

https://github.com/blog/1867-github-pages-now-runs-jekyll-2-2-0
Use "where" filter for building history layout.

Jekyll 2.0.0 required.
Revert "Remove subdirectories from _data/."

We can bring this back now that we're on Jekyll 2.2.0.

This reverts commit 89033d4800d281bd3b37f9158288a0eb8ebe80b7.
Merge pull request #77 from kevinwang/jekyll-2.2.0-refactors

Jekyll 2.2.0 refactors