Add support for doing eval in terms of a series of pairwise comparisons

The LFW protocol defines the testing procedure as 10-fold cross validation over a series of pairwise comparisons, i.e. not over full comparison matrices. 

This branch adds minimal support for this protocol, by adding sigsets for the testing procedure, and pairwise comparison functions to br (-pairwiseCompare, -makePairwiseMask) which can be used to generate N by 1 comparison matrices and corresponding masks based on lists of comparisons.