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/phone-input-view.js

@@ -17,12 +17,11 @@ var Whisper = Whisper || {};
 
 (function () {
     'use strict';
-    Whisper.PhoneInputView = Backbone.View.extend({
+    Whisper.PhoneInputView = Whisper.View.extend({
         tagName: 'div',
         className: 'phone-input',
+        template: $('#phone-number').html(),
         initialize: function() {
-            this.template = $('#phone-number').html();
-            Mustache.parse(this.template);
             this.render();
         },