Default image element

When the `imageField` option is specified, we'll always show the `angucomplete-image-holder` element. And if an image is found, we'll render the `angucomplete-image` element, if not, we'll render the `angucomplete-image-default` element.

The user can style the `angucomplete-image-default` element to provide a default background image.

Also, symlink the assets in the `example` directory.