This work has been supported by by NSF awards 1546393, 1302522, 1322278, 1111750, 0943168, 1318808 and 1552944, the Science of Security Lablet (H9823014C0140), the U.S. Department of Defense through the Systems Engineering Research Center, LARC (Living Analytics Research Centre), and a grant from the Alfred P. Sloan Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or other funders of this work.