DRY up a common view pattern

Define a Whisper.View base class that automatically parses and renders
templates and attributes defined by the subclass. This saves us a good
number of lines of code as well as some marginal memory overhead, since
we are no longer saving per-instance copies of template strings.

js/views/message_view.js

@@ -33,12 +33,9 @@
     });
 
 
-    var ContentMessageView = Backbone.View.extend({
+    var ContentMessageView = Whisper.View.extend({
         tagName: 'div',
-        initialize: function() {
-            this.template = $('#message').html();
-            Mustache.parse(this.template);
-        },
+        template: $('#message').html(),
         className: function() {
             if (this.model.get('delivered')) { return 'delivered'; }
         },