Model node transformations and other improvements

[shunter]

This supersedes #101 including all of Greg's changes, and additionally starts working towards wrangling this mess of a project into shape.

CZML now supports the general concept of key-value data, for example:

{
  "model":{
    "nodeTransformations":{
      "node1":{
        "scale":{
          // ...
        }
      }
    }
  }
}

This is generated by specifying "additionalProperties" in the jsonschema, which essentially associates structure with any property, rather than naming specific CZML properties. jsonschema allows for constraining the name of additional properties, but for this use case this is unnecessary.

Along the way I significantly refactored the GenerateFromSchema code. I also took a pass through the existing schema and fixed and improved documentation in a number of places. The concept of nextTime/previousTime has never been supported, and has been removed. Interpolation/extrapolation-related properties were not documented and have been added. The "axes" property on orientation has never been supported, and has been removed. Boolean properties were not allowed to be references, which was an oversight, and has been corrected.

For completeness, additional CZML changes from Greg's PR include a Boolean property runAnimations to control whether animations run on load.