Implemented optional<> and string_view<>

Conflicts:
	Platform/Apple/ePub3.xcodeproj/project.pbxproj
	Platform/Apple/ePub3.xcodeproj/project.xcworkspace/xcshareddata/ePub3.xccheckout
Implemented C++17 classes:
 - executor (R3)
 - optional (R5)
 - string_view
 - future/promise/async/packaged_task with aFuture.then(someFn)

Conflicts:
	Platform/Apple/ePub3.xcodeproj/project.pbxproj
	Platform/Apple/ePub3.xcodeproj/project.xcworkspace/xcshareddata/ePub3.xccheckout
Merge branch 'develop' of github.com:readium/readium-sdk into develop

* 'develop' of github.com:readium/readium-sdk:
  moved buffer allocation outside of loop
  fixed xmlFreeDoc crash by de-registering the libxml2 bindings right after unpack (see __resetLibXMLOverrides)
  debugging: filtermanager is nil
  restored segfaulting code
  fixed bad MO merge
Fixed? resource deallocation race in Unit Tests.
Custom xmlNode(?:R|Der)egister functions now call their predecessors.
Added ContentModule spec PDF.