Add JSON protocol support for groups.

json_reporter.schema.json

 {
   "$schema": "http://json-schema.org/draft-04/schema#",
   "description": "An event emitted by the test package's JSON reporter",
 
   "definitions": {
     "Test": {
       "required": ["id", "name", "metadata"],
       "properties": {
         "id": {"type": "integer", "minimum": 0},
         "name": {"type": "string"},
         "metadata": {"$ref": "#/definitions/Metadata"}
       }
     },
 
+    "Group": {
+      "required": ["id", "name"],
+      "properties": {
+        "id": {"type": "integer", "minimum": 0},
+        "parentID": {
+          "oneOf": [{"type": "integer", "minimum": 0}, {"type": "null"}]
+        },
+        "name": {
+          "oneOf": [{"type": "string"}, {"type": "null"}],
+        },
+        "metadata": {"$ref": "#/definitions/Metadata"}
+      }
+    },
+
     "Metadata": {
       "required": ["skip", "skipReason"],
       "properties": {
         "skip": {"type": "boolean"},
         "skipReason": {
           "oneOf": [{"type": "string"}, {"type": "null"}]
         }
       }
     }
   },
@@ skipping 20 matching lines @@
     {
       "title": "TestStartEvent",
       "required": ["test"],
       "properties": {
         "type": {"enum": ["testStart"]},
         "test": {"$ref": "#/definitions/Test"}
       }
     },
 
     {
+      "title": "GroupEvent",
+      "required": ["group"],
+      "properties": {
+        "type": {"enum": ["group"]},
+        "group": {"$ref": "#/definitions/Group"}
+      }
+    },
+
+    {
       "title": "PrintEvent",
       "required": ["message"],
       "properties": {
         "type": {"enum": ["print"]},
         "message": {"type": "string"}
       }
     },
 
     {
       "title": "ErrorEvent",
@@ skipping 25 matching lines @@
         "type": {"enum": ["done"]},
         "success": {"type": "boolean"}
       }
     },
 
     {
       "title": "FutureEvent",
       "properties": {
         "type": {
           "not": {
-            "enum": ["start", "testStart", "print", "error", "testDone", "done"]
+            "enum": [
+              "start", "testStart", "group", "print", "error", "testDone", "done"
+            ]
           }
         }
       }
     }
   ]
 }