NCX generation now supports invisible parts.
Maybe fix EPUB NCX validation error?
Revert "SVG files are now enabled."

This reverts commit 213c8a269db0a3c566baed35772813945ca53cf3.
Merge branch 'dev' of https://github.com/pressbooks/pressbooks into dev
Merge pull request #49 from bdolor/dev

replacing deprecated wp calls
Merge branch 'dev' of https://github.com/pressbooks/pressbooks into dev
php strict standards, redefining already defined constructor