Added state button in the Demo app

[patrice4github]

This state button calls playerAPI getPlayerState. It demonstrate that the callback never fires, or so I think.

The state button calls:

    NSLog(@"-----------> didTapStateButton");
    [[self.appRemote playerAPI] getPlayerState:^(id<SPTAppRemotePlayerState> state, NSError* error) {
        NSLog(@"-----------> getPlayerState returned");
    }];

I cannot see the second log message.

Further, in my app, I can call getPlayerState repeatedly and see it crash (too many callbacks in the stack).