Updated specifications for subtitle handling

Updated HTMLBook specifications for subtitle handling to be in accordance with best practices stipulated in "Common idioms without dedicated elements" in HTML5 specification (http://www.w3.org/TR/html5/common-idioms.html#common-idioms)

If a section has a subtitle, it now must be encapsulated along with the main title in a `<header>` block, as follows:

````html
<section data-type="chapter">
  <h1>This is the title</h1>
  <p>This is the subtitle</p>
````

Specification, Schema, and XSL tooling are all updated here in accordance with this change.