add constantTimeDispatcher and use it for strategic refreshes.

constantTimeDispatcher lets you poke a specific react component to do something
without having to do any O(N) operations.  This is useful if you have thousands
of RoomTiles in a RoomSubList and want to just tell one of them to update,
without either having to do a full comparison of this.props.list or have each
and every RoomTile subscribe to a generic event from flux or node's eventemitter

*UNTESTED*

src/ConstantTimeDispatcher.js

@@ -0,0 +1,62 @@
+/*
+Copyright 2017 Vector Creations 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.
+*/
+
+// singleton which dispatches invocations of a given type & argument
+// rather than just a type (as per EventEmitter and Flux's dispatcher etc)
+//
+// This means you can have a single point which listens for an EventEmitter event
+// and then dispatches out to one of thousands of RoomTiles (for instance) rather than
+// having each RoomTile register for the EventEmitter event and having to 
+// iterate over all of them.
+class ConstantTimeDispatcher {
+    constructor() {
+        // type -> arg -> [ listener(arg, params) ]
+        this.listeners = {};
+    }
+
+    register(type, arg, listener) {
+        if (!this.listeners[type]) this.listeners[type] = {};
+        if (!this.listeners[type][arg]) this.listeners[type][arg] = [];
+        this.listeners[type][arg].push(listener);
+    }
+
+    unregister(type, arg, listener) {
+        if (this.listeners[type] && this.listeners[type][arg]) {
+            var i = this.listeners[type][arg].indexOf(listener);
+            if (i > -1) {
+                this.listeners[type][arg].splice(i, 1);
+            }
+        }
+        else {
+            console.warn("Unregistering unrecognised listener (type=" + type + ", arg=" + arg + ")");
+        }
+    }
+
+    dispatch(type, arg, params) {
+        if (!this.listeners[type] || !this.listeners[type][arg]) {
+            console.warn("No registered listeners for dispatch (type=" + type + ", arg=" + arg + ")");
+            return;
+        }
+        this.listeners[type][arg].forEach(listener=>{
+            listener.call(arg, params);
+        });
+    }
+}
+
+if (!global.constantTimeDispatcher) {
+    global.constantTimeDispatcher = new ConstantTimeDispatcher();
+}
+module.exports = global.constantTimeDispatcher;