Merged in _qll/djangobb/_qll/remove_optional_import_parameter (pull request #42)

Fix Python 3.4.0 incompatibility in __import__ (remove optional default value).
Extract HTMLFilter base class from ExcludeTagsHTMLParser
Extract a base class from ExcludeTagsHTMLParser that collects filtered
HTML output as an HTML document is parsed.

The next patch will add a new subclass for adding attributes to HTML
tags.  The base class avoids duplicating code.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add DJANGOBB_NOFOLLOW_LINKS setting to add <a rel="nofollow">
User-submitted links are not trusted and should be marked rel="nofollow"
so that search engines know this.  This dissuades spammers from posting
links since it will not improve their search engine ranking.

Make rel="nofollow" optional in case someone prefers the old behavior.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Add test case for utils.add_rel_nofollow()
Both convert_text_to_html() and add_rel_nofollow() should add the
rel="nofollow" attribute to <a> tags.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>