Only display published browse categories on Browse#index.

Fixes #568 

![browse-admin](https://cloud.githubusercontent.com/assets/96776/2556556/a59cb7e2-b6d7-11e3-9aad-708961547195.png)

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![browse-index](https://cloud.githubusercontent.com/assets/96776/2556557/a9359176-b6d7-11e3-834a-aec04f2aa152.png)