juwang-make named class PortNumber public

In reef-webServer, PortNumber, MaxPortNumber and MinPortNumber are named parameters used in HttpServerImpl. Users can set those numbers if they don’t want to use default one. Although the port number set by client cannot be guaranteed if there is a conflict. 

Currently, those named parameter classes are not public. This PL is to change them into public so that to allow clients to set their preferred numbers if they wish.

This is for issue #1016
typo: "mim" -> "minimum"