added the ability to have possessive/genitive citations (e.g., Pacer and Suchow's paper on proselint)

[mpacer]

Add the ability to cite papers in a possessive form (which is otherwise impossible to do with natbib).

Fix drawn from: http://tex.stackexchange.com/questions/125690/bibtex-and-genitive-possessive-s-the-proper-way-to-obtain-kurans-1989-mo

really this should be built into natbib, but that hasn't been touched since 2010, so I'm not holding my breath, and I don't want us to be shipping our own incompatible version of natbib, so this seemed like the most sensible place to put it.

[mpacer]

I didn't add the xstring extension to handle ending "s"es because I didn't want to introduce another dependency. But if that's ok, then I'll add that as well.

[mpacer]

hmmm not sure why that broke