Add eigenvector centrality function

[jhollway]

Original report by James Hollway (Bitbucket: jhollway, GitHub: jhollway).

---

[henriquesposito]

I am having a bit of trouble finding a consistent, as well as, translating the mathematical equations for eigenvector in two-mode networks. There are several mentions of it across the literature (Borgatti and Everett 1997; Borgatti and Halgin 2011; Faust 1997; among others). I think Faust (1997) is presents the better discussion for eigenvector for two-mode. I will keep on working and trying, just an update on where I am and why this is taking so long to finish.