I've half built a tool to do this using PhantomJS, and a really half-baked API. The image grab part is easy. The diffing is what's hard... well sort of.

What I tried and what this site appears to do is a straight pixel change detection, which fails to account for how important that change is. Minor things change on a site all the time it's catching major breaking changes that's hard, say a CSS rule change that looks fine on desktop but ruins the site on mobile.