title: Passing variable to directive's attribute
body: <p>In AngularJS, how can I use a variable within an attribute of a directive?</p>
  
  <p>Without any directives, this work fine:</p>
  
  <pre><code>&lt;a 
      href="#/fruits/{{ fruit.short }}/details" 
      title="Back to Fruit details"&gt;
      Back
  &lt;/a&gt;
  </code></pre>
  
  <p>Now with directive, this does not work:</p>
  
  <pre><code>&lt;backButton 
      href="#/fruits/{{ fruit.short }}/details" 
      title="Fruit details"&gt;
  &lt;/backButton&gt;
  
  
  MyApp.directive( 'backbutton', function() 
  {
      return {
          restrict: 'E',
          link: function( scope, element, attrs ) 
          {
              var href    = attrs.href;
              var title   = attrs.title;
  
              console.log( "href = " + href );    // undefined
              console.log( "title = " + title );  // Fruit details
  
              element.html('&lt;a href="' + href + '" title="Back to ' + title + '"&gt;Back&lt;/a&gt;');
          }
      };
  });
  </code></pre>
  
  <p>The directive itself works fine for e.g. <code>href="#/fruits/novariableused"</code>. But as soon as I use a variable in the <code>href</code> attribute, its value becomes <code>undefined</code>.</p>
  
  <p>How can I fix this ?</p>