Rank-Biased Overlap (RBO): a similarity measure for indefinite ranked lists

This code implements the RBO metric, as described in:

@article{wmz10:acmtois,
    author = "Webber, William and Moffat, Alistair and Zobel, Justin",
    title = "A similarity measure for indefinite rankings",
    journal = "ACM Transactions on Information Systems",
    year = {2010},
}