ROC area under the curve for binary classification

Implementation of receiver-operator-curve area under the curve goodness of fit measure.

JIRA:
https://spark-project.atlassian.net/browse/MLLIB-23

References:
http://web.engr.oregonstate.edu/~tgd/classes/534/slides/part13.pdf, slide 11.
http://cs.ru.nl/~tomh/onderwijs/dm/dm_files/roc_auc.pdf (@srowen)

Incubator-spark pull request (old):
https://github.com/apache/incubator-spark/pull/550