Raises error if zero or negative value passed to lgamma
Minor C style adjustments
DOC: update release howto
Use wheelhouse-uploader to upload Windows wheels

Closes #11
Flake8 is needed for running tests with tox
Reference additional files in MANIFEST.in for Debian packaging

While pbr takes care of including extra files (other than .py)
when there is git repository information around, Debian's pybuild
does not have access to that information.
CI: Test against Python 2.7 on Travis