---- Add Ipv4Address operator <, to allow Ipv4Address to be used in std::set; Closes #39. user: Gustavo J. A. M. Carneiro <gjc at inescporto.pt> files: src/node/ipv4-address.cc src/node/ipv4-address.h url: http://code.nsnam.org/ns-3-dev/rev/a3f6b5783790