Changes for Third Edition

Please list possible changes for the third edition here.

- [x] Convert book source to RMarkdown

- [x] Use [rio](https://github.com/leeper/rio) for data importing and exporting.

- [x] Follow [system vs. package naming conventions](http://andrewgelman.com/2016/01/14/rstanarm-and-more/#comment-259425).

- [x] `repmis` no longer supports downloading data from Dropbox due to changes in the Dropbox API. Remove this discussion and the package from the book.

- [x] Remove `ZeligBayesian` as it is no longer needed by `Zelig`. Also remove example in 9.3.5 with the package. Changes in `Zelig` have broken the example.

- [x] [R Markdown Notebooks](http://rmarkdown.rstudio.com/r_notebooks.html)

- [x] Discuss databases

- [x] [Google R style guide link update](https://github.com/christophergandrud/Rep-Res-Book/issues/76)

- [x] Update broken dropbox link (though maybe just use different data example) https://github.com/christophergandrud/Rep-Res-Book/issues/78

- [x] [installr](https://cran.r-project.org/web/packages/installr/index.html)

- [x] Discuss Jupyter notebooks

- [x] Use pacman for package installation in Front Matter: used `xfun::pkg_attach2()` instead as it is installed with *knitr*

- [x] Use [here](https://github.com/r-lib/here)