Addded support for nesting <img> tags as children of <section> elements as well as Part <div>s.

@adamwitwer, @nelliemckesson,

Per our discussions on Monday, I've updated the Schema such that it is valid HTMLBook to nest `<img>` tags directly as children of sectioning elements. So it's now fine to do the following:

````html
<section data-type="sect1">
  <h1>Pet photos</h1>
  <p>Here is a picture of my cat:</p>
  <img src="kitty_yarn.jpg"/>
</section>
````

The above can be used for all informal figures. For formal figures, as we discussed, you can put a caption at either the beginning or end. This is fine:

````html
<figure>
  <figcaption>Picture of my cat</figcaption>
  <img src="kitty_yarn.jpg"/>
</figure>
````

And so is this:

````html
<figure>
  <img src="kitty_yarn.jpg"/>
  <figcaption>Picture of my cat</figcaption>
</figure>
````