lecture12: budget constraint model and branch and bound
http://mat.gsia.cmu.edu/orclass/integer/integer.html
http://en.wikipedia.org/wiki/Branch_and_bound
http://mathworld.wolfram.com/BranchandBoundAlgorithm.html
http://www.rpi.edu/~mitchj/papers/leeejem.html
http://www.mat.univie.ac.at/~neum/glopt/software_g.html