CZML Model support.
Merge branch 'master' into models

Conflicts:
	DotNet/CesiumLanguageWriter/Generated/PacketCesiumWriter.cs
	Java/CesiumLanguageWriter/translatedSrc/cesiumlanguagewriter/PacketCesiumWriter.java
	Schema/Packet.jsonschema
Update model properties.
Merge branch 'master' into models
Cleanup doc.
Changes after review.

Get rid of Image and Gltf writers, instead consolidate them into a single Uri writer.
Delete no longer needed Java files.
Rename `image` and `gltf` properties to `uri`.
Revert "Rename `image` and `gltf` properties to `uri`."

This reverts commit 2df6afb4a802807e4d8c12b9b24a781345d59a64.
Merge pull request #60 from AnalyticalGraphicsInc/models

Models