Fix an issue where the client failed to inject into some VitalSource book
chapters, by revising the tests used to decide whether a content frame should
have the client injected into it. The previous logic worked for most EPUBs and
PDF books, but failed for:

 - EPUB chapters which had no paragraph content (eg. only an image,
   table or list)

 - "Great Book" format EPUBs, which are used for many of the free
   classic texts available in VitalSource. These use `<div
   class="para">` elements for paragraphs instead of `<p>` elements
   (sigh).