Fix UnicodeError in HTML output

 * Alter `test_unicode_append` so that actual unicode characters outside the ASCII bytestring range are tested.

 * Make sure output of `render` in markup.py is unicode

 * Add wrapper around output of `export_set` in _html.py so that unicode characters are output.
Make sure codecs module loaded for all versions of Python
Merge pull request #63 from jsdalton/fix_unicode_error_in_html_output

Fix unicode error in html output