Ruby implementation of the string similarity described by Simon White at: www.catalysoft.com/articles/StrikeAMatch.html

                       2 * |pairs(s1) INTERSECT pairs(s2)|
  similarity(s1, s2) = -----------------------------------
                           |pairs(s1)| + |pairs(s2)|

e.g.

                                            2 * |{FR, NC}|
  similarity(FRANCE, FRENCH) = ---------------------------------------
                               |{FR,RA,AN,NC,CE}| + |{FR,RE,EN,NC,CH}|

                             = (2 * 2) / (5 + 5)

                             = 0.4

  WhiteSimilarity.new.similarity("FRANCE", "FRENCH")