Error infrastructure now in place.
Next step: make the core library use it EVERYWHERE.
Merge branch 'feature/refcounted' into feature/errors

* feature/refcounted:
  Fixed some issues with IRI class.
  Fixed a bug in SpineItem property parsing.
  declared NavigationElementPtr, NavigationTablePtr for consystency

Conflicts:
	Platform/Apple/ePub3.xcodeproj/project.pbxproj
Merge branch 'feature/refcounted' into feature/errors

* feature/refcounted:
  Updated header licenses.
Updated license headers.
Partial conversion to spec error handlers.
Error handling and supporting unit tests.
Not at 100% coverage yet.
More error handling and package tests.
More package validation and tests.
Fixed an error in swap_traits for non-libc++ implementations.
Merge branch 'release/0.5' into develop

* release/0.5:
  Fixed handling of spine item properties. Also correctly turns 'rendition:orientation-landscape' into 'rendition:orientation' = 'landscape' and similar for other FXL properties.
Merge branch 'feature/errors' into develop

* feature/errors:
  Fixed an error in swap_traits for non-libc++ implementations.
  More package validation and tests.
  More error handling and package tests.
  Error handling and supporting unit tests. Not at 100% coverage yet.
  Partial conversion to spec error handlers.
  Updated license headers.
  Error infrastructure now in place. Next step: make the core library use it EVERYWHERE.

Conflicts:
	ePub3/ePub/package.cpp
Merge branch 'release/0.6'

* release/0.6:
  Fixed an error in swap_traits for non-libc++ implementations.
  More package validation and tests.
  More error handling and package tests.
  Error handling and supporting unit tests. Not at 100% coverage yet.
  Partial conversion to spec error handlers.
  Updated license headers.
  Error infrastructure now in place. Next step: make the core library use it EVERYWHERE.