Support image uploads to a default directory from the admin interface

It would be great to be able to upload images from within the admin editing interface. Inspiration can probably be taken from [the way GitHub issue attachments work](https://github.com/blog/1347-issue-attachments).

The default upload directory can be something like `/files/<year>/<month>/<name>`, but configurable.

Thoughts:

1. Orphaned uploads if you use lots of images while drafting?
2. Having a separate view of, e.g., `files/` to browse stuff outside of the editing interface?
3. If you draft on 2012/12 and publish on 2013/01, the file should probably end up in 2013/01 to keep it related to the post it's on. Temporary holding area for "unpublished" images?