Solving issue #110: Epub only contain the last chapter
Merge pull request #111 from ploum/master

Fixed the ePub generation for un-numbered books

If the book contents (e.g. chapters) doesn't define their numbers in the config.yml file, the resulting ePub book only contains the last item. Although this is a very uncommon scenario, the fix is simple and doesn't affect the overall performance.

Thanks @ploum for the pull request.