1. Importing "Code and Data" paper by Gentzkow and Shapiro (with permission).
2. Adding new layout for same.
3. Adding link to paper.
4. Modifying Makefile to rebuild when any HTML files in the `bib` directory are changed.
Merge pull request #404 from gvwilson/adding-gentzkow-shapiro

Adding "Code and Data" paper to website.