Make compatible with boost 1.73.0+ and gcc 4.4

[CamJN]

gcc 4.4 doesn't have support for the <functional> c++11 header, and relies on boost bind. But boost bind now tells you to use #include <boost/bind/bind.hpp> instead of #include <boost/bind.hpp>, and that requires scoping the boost::placeholders namespace fully, but has the benefit of allowing a cleaner namespace definition more like the c++11 one.