first cut of improving UX for deleting devices.

This adds a 5 minute auth cache to speed up the process of deleting
old devices.  It has the following nastinesses (mainly due to being
written on a flight whilst juggling kids):
 * the auth cache is done as context attached to MatrixChat.
   one could argue that it should be per-client instead, but we don't
   yet have multiple clients.
 * the auth cache is only maintained currently in DevicesPanelEntry
   (i.e. set & invalidated).  One could argue that it might be better
   maintained in InteractiveAuth.js or a dedicated cache object
   abstraction, but given the only use I can think of is when managing
   devices, perhaps this is good enough for now.

src/components/structures/MatrixChat.js

@@ -68,6 +68,7 @@ module.exports = React.createClass({
 
     childContextTypes: {
         appConfig: React.PropTypes.object,
+        authCache: React.PropTypes.object,
     },
 
     AuxPanel: {
@@ -77,6 +78,10 @@ module.exports = React.createClass({
     getChildContext: function() {
         return {
             appConfig: this.props.config,
+            authCache: {
+                auth: {},
+                lastUpdate: 0,
+            },
         };
     },