Lightbox overlay

- [x] Make sure we run the post processing when rebaking the posts
- [x] Add an overlay when hovering lightboxes:

![screen shot 2013-06-21 at 6 37 36 pm](https://f.cloud.github.com/assets/362783/688225/e770de7e-da90-11e2-9fe8-a0d51c273d00.png)