Change JSON model naming behavior.

- JSON model names now come from the file name
- The 'Name' field is now optional for the top-level instance
- Snapshot tests run way faster by executing Rojo directly instead of Cargo

rojo-test/build-tests/json_model_in_folder/folder/simple-model.model.json

@@ -1,5 +1,4 @@
 {
-  "Name": "Top-Level Value",
   "ClassName": "IntValue",
   "Properties": {
     "Value": 5