Add image attachments as images to the sitemap.

So... It works a bit like this....

I have a blog that attaches images as custom post meta. This may be multiple images that my blog then turns into a slideshow etc. I want these images showing up in my sitemap as they are infact images associated with the post, and although they aren't in the "content" per-say, they are actually displayed on the post with the text content. 

I'm unsure if you want to include this, or if you want to add a feature to turn it off and on. But I thought I'd create the pull request anyway :)