title: InvalidTemplateLibraryError on initial runserver using wagtail
body: <p>Everytime i do
  <code>python manage.py runserver</code>
  an error said</p>
  <blockquote>
  <p>Invalid template library specified.</p>
  <p>ImportError raised when trying to load 'wagtail.wagtailcore.templatetags.wagtailcore_tags': cannot import name _htmlparser</p>
  </blockquote>