use libepugen

[aoloe]

if it meets our needs, we should use libepugen!

• https://davetardon.wordpress.com/2014/06/22/introducing-epub-generator-for-librevenge/
• http://people.freedesktop.org/~dtardon/documentliberation/libepubgen-sample/
• http://sourceforge.net/p/libepubgen/