Revert "Switch from Kramdown to Pandoc."

This reverts commit d19ebc645fdf5a7d85d7997cac1c46e5f39f7019.
Add metadata to posts.
Update source line in Gemfile to https.
Update gems to latest versions.
Add subject (category/tags) to DC metadata.
Merge branch 'master' into feature/metadata
Switch to using Google Scholar tags for metadata.
Switch to ruby-{version,gemset} from rvmrc.
Update to Ruby 2.1.0.
Update all gems to latest versions.