Instead of Epub.open('path'), now use Epub('path'). This makes the
class more intuitive to use. This also removes the need for the
_original_epub_filepath argument which was simply awkward.
We now support read-only books with the read_only argument. The main
benefit is that we don't need to unzip the epub file, so if you are
going to be reading from epubs but don't need to write, setting
read_only can significately improve speed.
