Confidence of clustesr

.match now returns cluster confidence score. Relates to #26 

The score is the "cophenetic distance" of the cluster

"In a hierarchical cluster tree, any two objects in the original data set are eventually linked together at some level. The height of the link represents the distance between the two clusters that contain those two objects. This height is known as the cophenetic distance between the two objects."http://www.mathworks.com/help/stats/hierarchical-clustering.html

@jernsthausen, does this meet the need you identified?

@derekeder, @evz csvdedupe and dedupe-web will need to be updated to handle the new output of match 
We are getting numpy structured arrays, we could do the subtraction all at once, if we updated the tests.