Functions to return classifier metrics

[MilesMcBain]

I was linked to this by @brendanvr. Nice package! I was wondering if you would consider exporting the stuff in metrics.R? There is a real lack of support for scoring binary classifiers in modelr and these functions would be a good start! Unless you think they belong in a separate package for this purpose?

[adefazio]

Ok, I'll look into adding some comments and exporting them.