Make subunit2html.py py3k and python2 compatible.

* .../subunit2html.py: Open the html file for writing in binary mode so
that we can specify utf8 as the encoding. Stop trying to decode strings
that have already been preprocessed by the subunit parser. Treat them as
strings and encode to bytestring when writing to the output file.

Change-Id: I81f13794ad7b40dfbc8ad44d2a9a5acd855c0029
Merge "Make subunit2html.py py3k and python2 compatible."