First pass at polygon hierarchy support in data sources.
Add PolygonHierarchy object to code.

Used mostly for documentation, but can be instantiated with positions and holes.

Also deprecate `PolygonGraphics.positions` and add `PolygonGraphics.hierarchy` to replace it.
Merge remote-tracking branch 'origin/master' into polygongraphics-holes
Ongoing DataSource polygon hole work.

Update CHANGES.
Clean up some doc.
Make CzmlDataSource use `hierarchy` instead of `positions` (CZML still doesn't support holes).
Fix Entity.orientation.
Merge branch 'polygongraphics-holes' into implicit-properties

Conflicts:
	Source/DataSources/PolygonGraphics.js