URLs, citations, and footnotes

[trevorcampbell]

major changes:

• For data sets: provide citation for article where data originally released, provide URL to the data itself
• For R packages: link to main doc page + bib citation (usually @Manual)
• For non-code related references (eg books), just provide citation. bibtex can include URL if the resource is freely available.
• added citations for kmeans, best subset selection, forward selection, knn methods
• http -> https (except insideairbnb because that one doesnt work with https)
• added proper bib references for books
• cleanup additional resources sections links / citations etc

some incidental changes throughout not documented here.