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/message_detail_view.js

@@ -31,9 +31,9 @@
         },
         render_attributes: function() {
             return {
-                name       : this.model.getTitle(),
-                avatar_url : this.model.getAvatarUrl(),
-                conflict   : this.conflict
+                name     : this.model.getTitle(),
+                avatar   : this.model.getAvatar(),
+                conflict : this.conflict
             };
         }
     });