Support embedding a single note

Here's the embed code:

``` html
<div id="DC-note-138916" class="DC-note-container"></div>
<script src="//s3.amazonaws.com/s3.documentcloud.org/notes/loader.js"></script>
<script>
  dc.embed.loadNote('http://www.documentcloud.org/documents/1001227/annotations/138916.js');
</script>
```
