Test files, documentation source and others missing from source packages

[moreati]

Only testlib.py from the test suite makes it to PyPI

$  tar tf mitogen-0.3.5.tar.gz| grep test        
mitogen-0.3.5/tests/
mitogen-0.3.5/tests/testlib.py

this is contrary to what Django and ansible-core do. Fix will probably be an audit of the git repo and tweaks to MANIFEST.in.

Discovered during review of #1045