Add labels to the thumbnails in the 3 different viewers.

Closes #134 

### Example cn006dx2288

#### List
![cn006dx2288-list](https://cloud.githubusercontent.com/assets/96776/4985203/f1b03388-692a-11e4-8307-7a3efdccf4d3.png)

#### Gallery
![cn006dx2288-gallery](https://cloud.githubusercontent.com/assets/96776/4985204/f1b275a8-692a-11e4-9d6e-89c20ba561e5.png)

#### Horizontal
![cn006dx2288-horizontal](https://cloud.githubusercontent.com/assets/96776/4985205/f1c71ad0-692a-11e4-9872-8d75bebd0dc7.png)