This adds a new integration for the VitalSource Bookshelf reader. It is
comprised of two separate sub-integrations, one for the "container"
frame into which the client is initially loaded, and a separate one for
the "content" frame that displays the book content.

The container frame integration is responsible for injecting the client
into the content frame and re-injecting after a navigation. All the
annotation-related functionality is a no-op in this frame. The logic for
injecting the client depends on a stub method in the Guest class which
will be implemented separately.

The content frame integration mostly delegates to the HTML integration,
but it customizes the document metadata saved with annotations and also
prevents the VitalSource viewer's native selection controls from
interfering with Hypothesis's ones.