Screenshot Save for Snapchat pretty clearly does both of these things. To use the app, you have to log in with your SnapChat account (which automatically logs you out of the SnapChat app), and when a message comes in, you can choose to save it, unless it has already been viewed. The free version has pop up ads, and stores the images with a watermark within the app. Paying to upgrade the app lets you save messages to your Camera Roll at full size with no watermark.
Screenshot Save for Snapchat has been on the App Store since July 30th. We have reached out to developer Sepia Software and SnapChat for comment, but have so far not received a reply.