Add missing joke.py file to the funniest package

[davidkurkov]

The joke.py file appears to be missing from the funniest package (at the same directory level as command_line.py) - the package outline in this doc should be updated as well: http://python-packaging.readthedocs.io/en/latest/everything.html

[merwok]

But the setup.py defines only packages=['funniest'] (and the package exists), no py_modules at all.