added calls to add_dependencies for the sake of new ros users.
(though they are not strictly necessary in this package, as for some reason, the talker/listener example does not use the Num.msg.
for now, add genmsg dependencies.
(to be changed to foo_generate_messages_cpp when genmsg has been released9
removed unnecessary directory argument, users should rely on default
updated to latest catkin_pkg
updated comments