More icons

Thanks for the component. Here are some changes to make component accept any character as icon. Unfortunately custom characters doesn't work when it is not ASCII (see demo.html). Neither does Unicode chars like &#9786; or \u263A work, when used as expression in double mustache (line 38 of star-rating.html). Maybe needs a [filter](https://github.com/addyosmani/polymer-filters) for escaping?!.