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

@@ -0,0 +1,12 @@
+{
+  "ClassName": "IntValue",
+  "Properties": {
+    "Value": 5
+  },
+  "Children": [
+    {
+      "Name": "A Child",
+      "ClassName": "Folder"
+    }
+  ]
+}