Implement the "Revision History" widget

The PSD has a layer for items without a preview as well. This will show similar to how content without a preview shows in my ContentPreview branch.

![revisionhistory-markedup](https://f.cloud.github.com/assets/109850/577850/4e1c8f26-c842-11e2-9351-b805147c16b9.png)
![revisionhistory-nopreview](https://f.cloud.github.com/assets/109850/577851/4e33f9d6-c842-11e2-9fcc-3d6a779dc4b0.png)
![revisionhistory-withpreview](https://f.cloud.github.com/assets/109850/577852/4e505cfc-c842-11e2-8e75-de87ba45d298.png)