Fix download of attachments from messages with unicode subjects

[moba]

Fixes a problem with unicode subjects in messages:

PostboxDocument(msg_date=datetime.datetime(2017, 10, 20, 10, 58), msg_type=u'Bankpostkorb', is_new=True, url=u'/ptlweb/WebPortal?frame=content...', subject=u'Bankpostkorb F\xfcr Ihr Unternehmen: AGB-\xc4n...', postbox_page=u'4')

Access postbox
Access postbox page 4
Traceback (most recent call last):
  File "fetch.py", line 43, in <module>
    scraper.download_document(document, [directory])
  File "vbscraper.py", line 229, in download_document
    print("Download document {}".format(document.subject))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 14: ordinal not in range(128)