New theme option: image styles

This theme option lets you choose how to style images in entries and
static pages, basically any images from the media db that appear in
the content area of the blog.

Possible values: no border, simple border or shadow border.

Implemented using a class on #content, so you could even overwrite
.plain-images and/or .fancy-images in your user.css.
Default theme option imgstyle to plain images.