Ongoing CZML versioning work

1. Refactor how clock/name are processed.
2. Throw when the first packet is not a document object or is missing version.
3. Update specs.
update doc
Merge pull request #1945 from AnalyticalGraphicsInc/modelThrowOnLoadError

Model throw glTF file load error
Merge remote-tracking branch 'origin/master' into czml-cleanup
Merge branch 'czml-cleanup' into czml-versioning
fix typo
And CZML versioning tests and fix bugs uncovered by them.
Tweak CONTRIBUTING.md
Fix label collection.
Add image sub-regions to billboards.
Fix Sandcastle examples.
Merge remote-tracking branch 'origin/master' into gltf-8
Changes after review.
Fix Billboard and BillboardCollections tests. Remove TextureAtlasBuilder tests.
Fix CZML.
Merge pull request #1947 from AnalyticalGraphicsInc/czml-versioning

CZML versioning
add demo tag
Fix remaining tests.
Some clean up and add doc.
WebGL Globe JSON DataSource

1. Add a simple DataSource for WebGL Globe JSON.
2. Make geojson files use `.geojson` extension.
3. Add Sandcastle example.