allow port selection

I have developed an web app that needs to communicate via libspotify to the spotify server for more than one person. Since one instance of liesen/spotify-api-server can only serve data for one user, as the authentication is done when the instance is started, I needed to start multiple instances, on multiple ports.

This modification allows passing the port as a second parameter for the constructors of spotifyPlaylist and spotifyUser. If no port is provided, 1337 is used by default.

P.S. Sorry for the whitespace cleanup, if it's an issue I can probably remove it.