Issue 333: Show user Gravatars in Exclude Authors & Roles field

Fix #333 

![exclude results gravatar](https://f.cloud.github.com/assets/2076782/2407466/b8189b40-aa8c-11e3-86c3-ec7ae0102380.png)
![exclude selected](https://f.cloud.github.com/assets/2076782/2407465/b8170fe6-aa8c-11e3-852b-df72b85e2ca3.png)