optimize normalize_itext

http://manage.dimagi.com/default.asp?123086

@dannyroberts @czue @millerdev 

Final profile results: http://pastebin.com/SwTVEXeB
Interesting, `self.render()` returns an ASCII encoded string, but `parse_xml(...)` accepts a UTF-8 string. This works because ASCII is a proper subset of UTF-8.