Extract current spec version of Epub
Parse TOC (NCX file)

Implementation inspired by
https://github.com/joseph/peregrin/blob/master/lib/formats/epub.rb#L258-
L276
Typo
Fix issue when Epub doesn't have a NCX file (3.0)
Test Epub versions, plus some refactoring
testExtractingChaptersFromNcx
Merge pull request #1 from julien-c/master

Parse TOC (NCX file), plus a few small improvements