18 i child site inherits i imaging schema

We noticed that image scales were not available in the child site, so a url like http://localhost:8080/Plone/child-site/test-news-item/@@images/image/mini would throw an error. Let me know if anything looks amiss with this pull request. 

I think adding the interface to IChildSite is a good approach although perhaps there is a way to adapt a child site with the IImagingSchema instead of using inheritance?