Handle attachements with non-ascii filenames properly.

If an issue has an attachment with a non-ascii filename, the script currently fails with a UnicodeDecodeError in:
body = u'_From {author} on {date}_\n\n{body}'.format(**comment)

This is because type(comment['body']) == str, and it contains utf-8-encoded bytes from get_attachment's .encode('utf-8').

Example (with this fix):
https://code.google.com/p/clementine-player/issues/detail?id=151#c2
https://github.com/clementine-player/Clementine/issues/151#issuecomment-29982982