Python 3 compat, don't ignore credential_file arg, don't crash on 1st msg when chat was empty

[panzi]

• Python 3 compatibility:
  • raw_input -> input
  • decode bytes read from network with correct charset
  • HTMLParser -> html
  • urllib.urlopen -> urllib.parse.urlopen
  • new except syntax
  • relative imports
• In several function the credential_file parameter was ignored and a hard coded string was used instead.
• In case the chat was empty when the example script was started the script crashed when the first message arrived.
• Handle API errors in _json_request.
• git ignore vim .swp files and secret files.