Playlist support

Currently the module checks for `videoId` or `videoUrl` before loading. With the api you can create a playlist using `listType` and `list`. I did an initial implementation but adding an additional `typeof` check to the `stopWatchingReady` function. This works for me so far, how does this sound?

```
var stopWatchingReady = scope.$watch(
                function () {
                    return scope.utils.ready
                        // Wait until one of them is defined...
                        && (typeof scope.videoUrl !== 'undefined'
                        ||  typeof scope.videoId !== 'undefined'
                        ||  typeof scope.playerVars.list !== 'undefined');
                },
...
```

PS this module is great thanks!