title: Block tag are invalid
body: <p>I wrote block tag, when I try to cause it in template I got an error: </p>
  
  <blockquote>
    <p>Invalid block tag: 'endyoutube'</p>
  </blockquote>
  
  <p>My tag</p>
  
  <pre><code>from django import template
  
  register = template.Library()
  
  @register.tag
  def youtube(parser, token):
      link = parser.parse(('endyoutube', ))
      iframe = """&lt;iframe width="640" height="510" src="%s" frameborder="0" 
          allowfullscreen&gt;&lt;/iframe&gt;""" % link
      return YoutubeNode(iframe)
  
  class YoutubeNode(template.Node):
      def __init__(self, iframe):
          self.iframe = iframe
  
      def render(self, context):
          video = self.iframe.render(context)
          return video
  </code></pre>
  
  <p>template </p>
  
  <pre><code>{% load customtags %}
  {% youtube %}
      http://www.youtube.com/watch?v=yZt1qXnOOqU
  {% endyoutube %}
  </code></pre>
  
  <p>What is wrong in my tag? How could I fix it?</p>