Fixes a unicode import error in Nexpose

Due to a unicode conversion happening during lxml.etree processing the
web2py TAG function would throw an exception when trying to do a str()
conversion. Fixed by doing an var.encode('ascii', 'xmlcharrefreplace')
in html_to_markmin.

Some other misc fixes for better XML to HTML to MARKMIN conversion added
and some docstring tests.
Merge branch 'nx_unicode'