Add concern for handling extent labels and values.

Closes #267 

### Item results
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![books-video](https://cloud.githubusercontent.com/assets/96776/3389043/3d5dd568-fc90-11e3-988a-2867ab0862d9.png)

### Collection result
---
![archive](https://cloud.githubusercontent.com/assets/96776/3389061/6cb49856-fc90-11e3-8cfb-d7b115995cf3.png)

### Collection masthead
---
![collection-masthead](https://cloud.githubusercontent.com/assets/96776/3389068/769f87f4-fc90-11e3-93e1-05ae4a47f6e4.png)

### Access panel
---
![access-panel](https://cloud.githubusercontent.com/assets/96776/3389076/856878ae-fc90-11e3-837c-605be4cfdaf0.png)