Created a ReviewUIHook to allow extensions to register their 
own ReviewUIs into ReviewBoard.

Manually tested with XMLReviewUIExtension: http://reviews.reviewboard.org/r/3564/
- Successfully installed this extension with its own ReviewUI
- Succcesfully registered XMLReviewUI through XMLReviewUIHook
- Once installed, able to review XML file attachments via the "Review" button
- Color markup for the XML file attachment correctly rendered through pygments