feed: Use html.escape() instead of saxutils.escape()

feed: Use html.escape() instead of saxutils.escape()

html.escape() is new in Python 3.2 [1].  The effect is idential, but
this makes more sense semantically as we are escaping the entities to
use them in HTML, not in XML.

[1]: http://docs.python.org/3.2/library/html.html#html.escape

Signed-off-by: W. Trevor King <wking@tremily.us>