Add toggle-able Image preview to file list.

Closes #63 

Had some js scoping/styling-guide issues that I would be interested in hearing feedback on.

### Example (tn629pk3948)
![tn629pk3948](https://cloud.githubusercontent.com/assets/96776/4653928/001a2b00-54b2-11e4-8e0a-2fc97e557d97.png)

![image-preview](https://cloud.githubusercontent.com/assets/96776/4653930/028a7278-54b2-11e4-819b-7aadb420881e.gif)