Allow HTML in Field Settings Descriptions

It would be great to allow HTML in field setting descriptions. Then you can add things like links to the descriptions to save space.

### No HTML
![plugin-settings-no-html](https://f.cloud.github.com/assets/1065372/914624/9bec0688-fe42-11e2-814c-06b235ff0ff0.png)

### With HTML
![plugin-settings-with-html](https://f.cloud.github.com/assets/1065372/914625/a4c7ca94-fe42-11e2-85d2-c5fb26ce176a.png)