Added error_icon.png and spinner.gif images

Added error_icon.png and spinner.gif images to not make it dependent of
Plone resources.

converse.css

@@ -30,7 +30,7 @@
 }
 
 #connecting-to-chat {
-    background: url('/spinner.gif') no-repeat;
+    background: url(images/spinner.gif) no-repeat left;
     padding-left: 1.4em;
 }
 

converse.js

@@ -1818,7 +1818,7 @@
             $toggle.hide();
             $connecting.show();
             $connecting.html('Unable to communicate with chat server');
-            $connecting.css('background-image', "url('/error_icon.png')");
+            $connecting.css('background-image', "url(images/error_icon.png)");
             console.log("Connection Failed :(");
         }, this));