Use a hack to make sure that formstack resizes appropriately

This is a hack to make sure that formstack forms resize properly.

We should really use the script from interactives at http://interactive.guim.co.uk/libs/iframe-messenger/iframeMessenger.js but this is not available over SSL (not with a CDN anyhow) and formstack forms are all SSL. As a result I'm just grabbing the key techniques they are using in that file until such a time that they do have HTTPS.