Allow for embedding of YouTube videos

Both issue #27 and #28 require that YouTube videos may be embedded in content created
by non-admins. Embedding YouTube videos requires allowing iframe tags. In general, it
is a security risk to allow iframe tags. Therefore, we have taken a more conservative
route.

* Allowed the iframe tag in RICHTEXT_ALLOWED_TAGS in settings.py

* Create unit tests for the functionality we want: stripping invalid iframe tags.

* Create a filter function that checks iframe tags on whether they are indeed simple
embedded YouTube videos.

* Register this function in RICHTEXT_FILTERS to process all richtext output.

The demo data has been updated to reflect this change: Both the page 'Word actief!' and
the blog post 'Een mooie weblog' contain an embedded video.