Changes:
- Handle missing title from table of contents nodes that have children
- Change deprecation lxml check
- Throwing errors while writing EPUB is optional now
- Toning down metadata failure to an info log.
- Use posix module instead of the os.path when managing EPUB references
- lxml.etree._Element uses .text
- Using lxml text_content() on all nodes.
- Add compresslevel to ZIP failure
- Add direction to the epub Nav page only if it was set