Other 2x2 functions

Added diagnostic accuracy functions, and a function to calculate
Cohen’s kappa, two other uses of basic 2x2 tables that may be
encountered in Epidemiology.
Merge pull request #3 from elofgren/master

Merge from @elfogren. "Added diagnostic accuracy functions, and a function to calculate
Cohen’s kappa, two other uses of basic 2x2 tables that may be
encountered in Epidemiology."