Don't allow call numbers w/o shelfkeys to be browsable.

Closes #952 

### 5512697 Before
![5512697-before](https://cloud.githubusercontent.com/assets/96776/4397887/66ba473c-444a-11e4-957f-2c8d3beab6bc.png)

### 5512697 After
![5512697-after](https://cloud.githubusercontent.com/assets/96776/4397888/66d32de2-444a-11e4-9da7-1f98ca58f72b.png)