Print URLs, admonition headings, EPUB 2 compatibility, and sidebar XREFs

Four key updates to the HTMLBook XSL stylesheets:

* New core XSL handling for rendering `href` values for external hyperlinks separately in the output, which is useful for print outputs or any other medium in which clicking hyperlinks is not possible. Handling for rendering `href` URLs in output is enabled by setting the `url.in.parens` parameter to 1. When `url.in.parens` is enabled, markup like the following:

````html
Where EPUB semantics come up short, HTMLBook
falls back to <a href="http://www.docbook.org/">DocBook</a> semantics
````

Will be transformed by XSL to:

````html
Where EPUB semantics come up short, HTMLBook
falls back to <a href="http://www.docbook.org/">DocBook</a><span class="print_url_in_parens"> (<span class="print_url">http://www.docbook.org/</span>)</span> semantics
````

The two classes on `<span>`s ("print_url_in_parens" and "print_url") can be used for CSS styling. 
Note that the logic in the XSL will compare the value of the hyperlink text node against the value of the `href`, and will not print out the URL in parens after if they both contain the same URL. In other words, the following markup:

```html
The HTMLBook spec can be found at <a href="http://oreillymedia.github.io/HTMLBook/">http://oreillymedia.github.io/HTMLBook/</a>
````

Will be left as is when the XSL is run, even when `url.in.parens` is enabled.

* New parameter added called `add.title.heading.for.admonitions`. When this param is enabled, headings (`<h6>`s) will be autogenerated for admonitions (`<div>`s with `data-type` of `note`, `tip`, `warning`, `caution`, or `important`) that do not have an `<h1>`-`<h6>` child already. For example, the following markup:

````html
<div data-type="note" class="note">
  <p>Please take note of this important information.</p>
</div>
````

Will be transformed to:

````html
<div data-type="note" class="note">
  <h6>Note</h6>
  <p>Please take note of this important information.</p>
</div>
````

* Improvements to handling triggered when `html4.structural.semantics` parameter is set to 1. Rather than convert `<section>`, `<figure>`, and `<aside>` directly to `<div>`, the handling now preserves the original HTML5 structural semantic element (for better accessibility in EPUB 3 readers) but adds a "fallback" `<div>` child that takes the `id` attribute to ensure intracontent hyperlinks and CSS styling work in reading systems (e.g., EPUB 2 readers) that do not support HTML5. For example, the following markup:

````
<section data-type="chapter" id="chapter01" ...>
  <header>
    <h1>Chapter 1. HTMLBook Markup</h1>
  </header>
  <p>This chapter describes...</p>
</section>
````

Will be transformed to:

````
<section data-type="chapter" ...>
   <div class="chapter" id="chapter01">
        <header>
          <h1><span class="label">Chapter 1. </span>Chapter 1. HTMLBook Markup</h1>
        </header>
        <p>This chapter describes...</p>
   </div>
</section>
````

* Added missing handling so that gentext for XREFs to `<aside>` elements (e.g., sidebars) would generate properly.