Added ePub processing to convert files to Baker's Hpub format.

Allows adding ePub packages to your shelf.json manifest. On first opening the ePub file, it scans for the book.json file. If it is not found, then we check for the ePub container.xml file before failing.

If it is an ePub file, scan the OPF manifest for package contents, and create a book.json file. (On subsequent opens, then it will parse this book.json file as normal.)

To do: Create index.html from ePub contents, and optionally provide for overriding the ePub stylesheet with custom CSS.