Default avatar support

Fixes #264

Implement the equivalent of java's String.hashCode on the conversation model.
Change avatar template and attributes. Use css classes for colors.

js/views/new_group_update_view.js

@@ -67,7 +67,7 @@
         render_attributes: function() {
             return {
                 name: this.model.getTitle(),
-                avatar_url: this.model.getAvatarUrl()
+                avatar: {url: this.model.getAvatar()}
             };
         },
         send: function() {