Fix zip paths to use correct path separator

Fix aerkalov/ebooklib#56: ePub extraction not working on Windows.
Using os.path for reading zip items is problematic on Windows because
it uses backslash as a path separator (os.sep is '\\').
Fixed #58 - Add basic documentation
Merge branch 'path'
Allow overriding modification time for ePub document

This allows for reproducible ePub contents for testing
Merge branch 'master' of github.com:aerkalov/ebooklib