update iarc icons, clean up content ratings css (bug 950233)

Have a map in the content ratings constants that maps labels to icon paths, simplifying the CSS.

Add new Generic/CLASSIND/PEGI icons.

![screen shot 2013-12-13 at 3 37 14 pm](https://f.cloud.github.com/assets/674727/1747187/a8a73a7a-6452-11e3-922f-950b5700d913.png)

![screen shot 2013-12-13 at 3 39 03 pm](https://f.cloud.github.com/assets/674727/1747188/aa7c1564-6452-11e3-9576-8cdbed32d0d2.png)