add usk rating refused to iarc client (bug 931036)

Problem:

- the IARC response was changed from the spec
- didn't have the icon
- wasn't mapping to the Rating Refused rating

Solution:

- add icon
- use labels instead of iarc_name for icon slugs
- have labels for ratings that don't have ages (rating pending, rating refused)

Test Suites Ran:

- constants

Screenshots:

![screen shot 2013-12-19 at 3 31 30 pm](https://f.cloud.github.com/assets/674727/1787404/b7265f1a-6905-11e3-8c9a-e3454f1fe2a2.png)