Split apart @larsvoigt's work that adds support for next/previous button visibility toggling
Merge pull request #100 from readium/feature/prev-next-button-visibility-lars

Split apart @larsvoigt's work that adds support for next/previous button...
Fixes #181 in the readium-js-viewer project
function wasn't renamed in previous refactoring
fix for https://github.com/readium/readium-js-viewer/issues/181
(missing explicit content direction when page progression direction is explicit to RTL)
fixes #103 (audio soundtrack epub:type values)
https://github.com/readium/readium-js-viewer/issues/164 Removed previous attempted fix (thanks Ryan!), added missing pause() instruction to let the Media Overlay player resume playback after the skipped elements (which may trigger a switch to the following SMIL).
minor changes to polish-up #79 (pagination API): removed obsolete param doc, hiding spine.direction internals by calling the existing API method, updated Media Overlays comments to reflect API change in ReadiumSDK.Models.CurrentPagesInfo
ReadiumSDK.Models.CurrentPagesInfo constructor was not called with correct parameters! (follow-up to #79, no need for a separate issue)
Fix fotnt size option not working with author-provided CSS