Link prediction algorithms #2

This is the sequel of PR #1117. I added Jaccard coefficient, Adamic-Adar index, and preferential attachment score as described [here](http://www.cs.cornell.edu/home/kleinber/link-pred.pdf).

I am thinking of adding Katz score too, probably utilizing the existing `katz_centrality` function, but I am quite busy at the moment. If there's anyone who could help that'll be great. I don't really understand the other algorithms mentioned in the paper so I won't implement it now.