Improved evaporation spectrum sampling algorithm

This pull requests implements a new sampling algorithm for the evaporation spectrum as outlined in a recent [talk by Forrest Brown](http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-14-27694). The motivation for the algorithm is to improve the rejection efficiency when the restriction energy is very low. The rejection efficiency for current algorithm can in theory go to zero, whereas the new algorithm has a minimum efficiency of 50%.