Add /iframe URL for embedding object HTML in an iframe.

### Example iframe
    <iframe src="http://127.0.0.1:3000/iframe?url=http://purl.stanford.edu/view/tn629pk3948"
            width="400px"
            height="500px"
            frameborder="0">
    </iframe>

Closes #127
@jkeck Looks good :+1: 

Minor feedback - Google's CDN for jQuery (`<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>` ) is preferable because most users might already have a cached copy of the library.