only assign book.cover if coverPath is available, fixes #158
Changed unarchiver parseFromString type to text/xml, handle blob links in uri and replace, fixes #152
Built, adding to v0.2.3
Support nav item with muliple properties, fixes #124
replaced zip.js with JSZip
Merge branch 'master' into gh-pages
Merge branch 'gh-pages' of https://github.com/futurepress/epub.js into gh-pages