Add key property and remove duplicated code.

[Kurun-pan]

You need to be able to specify a key to identify the widget. And I removed duplicated import code.

[samu-developments]

I think this closes #30, at least without a key my rating widget would not update when I passed it a new score. I had a situation where I needed to async fetch the user's (possible) current rating, and then later update the rating. Thanks!