Factor-out GroupStore and create GroupStoreCache

In order to provide feedback when adding a room to a group, the group summarry store needs to be extended to store the list of rooms in a group. This commit is the first step required.

The next step is to get the GroupRoomList listening to updates from GroupStore and expose the list of rooms from GroupStore.

(We're running out of words to describe the hierachy of things that store things)

src/stores/GroupStoreCache.js

@@ -0,0 +1,37 @@
+/*
+Copyright 2017 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import GroupStore from './GroupStore';
+
+class GroupStoreCache {
+    constructor() {
+        this.groupStores = {};
+    }
+
+    getGroupStore(matrixClient, groupId) {
+        if (!this.groupStores[groupId]) {
+            this.groupStores[groupId] = new GroupStore(matrixClient, groupId);
+        }
+        return this.groupStores[groupId];
+    }
+}
+
+
+let singletonGroupStoreCache = null;
+if (!singletonGroupStoreCache) {
+    singletonGroupStoreCache = new GroupStoreCache();
+}
+module.exports = singletonGroupStoreCache;