Handle post images in a principled way

[BartMassey]

Right now, an image foo.png can be created in site/posts and inserted in a post by referencing the root-relative path /posts/foo.png. This means that the images are stirred in with the posts in a haphazard way, and that the site must be set up with its root at /site.

Probably should add a site/media directory at the very least.

See the closely related Issue #32.

[BartMassey]

On my personal blog I've now just moved images into a site/media directory and called it a day. I think there's more to be done here, but it seems to work for now.