Updated book title requirements in XML Schema and corresponding XSL handling for EPUB

The "title" in the `<head>` is now the canonical location where the book title should be stored. Including an `<h1>` at the beginning of the `<body>` is now optional, and HTMLBook content will validate against the schema without it. 

htmlbook-xsl logic for EPUB (to generate title for OPF and NCX) has been updated accordingly, to look for title in `<head>` first and then in heading child of `<body>`.

@adamwitwer and @nelliemckesson, let me know if anything else is needed to facilitate styling