Small improvements for pygpghttpd

Hi!

1. Shebang: I'm completely new to python, but it seems like "python" defaults to python2 on most systems. Hope it's okay to make it say "python3".
2. Query string: I was wondering at the workaround for "+"-encoded blankspaces which didn't work for me, so I replaced the whole query string parser with the one included in urllib.parse -- which also handles the blankspace encoding along the way.
3. example.htm: When listing all the keys, only the last one was displayed.