Provide option to export to ePub

[hughrun]

ePub is the worldwide standard format for flowable offline electronic books. It provides a much better user experience than PDF, because the content is responsive to the screen size by default, an ultimately it is just an XML wrapper around the standard web trio of HTML, JS and CSS. Providing an ePub export option in addition to the PDF option already available would make GitBook a much more attractive option for creating more "book like" sites, such as open educational resources, for which I otherwise think GitBook is a really strong candidate.

There are several JavaScript libraries available for converting HTML or markdown to ePub.