Fixes issue 164 in the readium-js-viewer project. Skippability not working.
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)
restored Ryan's original code as per #181 and https://github.com/readium/readium-shared-js/commit/e41a3cc4aa781aad7487a3228817c789f0e2b465
Revert "restored Ryan's original code as per #181 and https://github.com/readium/readium-shared-js/commit/e41a3cc4aa781aad7487a3228817c789f0e2b465"

This reverts commit 65cf5f88483a0947e797a570ece91e71f5cc46e4.
Fixes #108
(non-linear spine item / document pages navigation)
Fixes #111 set a max height on svg elements just like we do for images
Modify rectangle-based visibility detection so it supports vertical writing mode.
Attempt at fixing findPageByRectangles for vertical writing mode
Second attempt at fixing findPageByRectangles for vertical writing mode
(works but confused about the requirement of RTL checks)
Remove disabled code for RTL handling when in VWM.
Special handling of RTL is not needed in this case.
adjusted CSS column algorithm in reflowable view to take into account single-column vertical writing mode (user setting has no effect, max text width corresponds  2x single-column threshold)
Merge pull request #113 from readium/feature/vertical-writing-client-rects

Visibility computations by client rectangles, vertical writing mode (column layout) #87