ePub 3 support added

* Added: ePub 3 support. The ePub 3 still contains the NCX for backwards
combatability to ePub 2 readers, though there is no gurantee that it'll
work on all readers.
*  The EPub class constructor now takes arguments for epub version,
EPub::BOOK_VERSION_EPUB2 or EPub::BOOK_VERSION_EPUB3
*  The other new parameters on the EPub constructor are for default
language and writing direction, used on the generated ePub 3 TOC.
* Added: Ability to reference sub chapter id's
*  ->addChapter($chapterName, $chapterLink); where the link must contain
the #id.