more pythonic argument parsing in respotify client

Hi there Hexxeh,

I cleaned up the respotify client to use __name__ == "__main__" as the entrance point and in addition I've also used argparse to grab the arguments off the command line which is the more standard way to do it.

As I'm using argparse I also chucked getpass in there so you don't have to stick the password into your command line history unless you explicitly want to.

I've got some more changes but will package them up appropriately.

Cheers
Andrew