add button to edit media screen

Adds a "Regenerate Thumbnails" button to the Edit Media submit metabox.

When performing site updates for clients, I often find myself using enable-media-replace to update an image, followed by an immediate regenerate-thumbnails. After replacing media, I find myself on the Edit Media page, but unfortunately must head back to the Media list to find and regenerate the images. It is good to just have this action available right here:

![regenerate-thumbnails-edit-media](https://cloud.githubusercontent.com/assets/824344/3018784/f2188a4e-df89-11e3-85bc-6bf2446e1afb.gif)