Automatically deploy to PyPI when tags are created

[danielhers]

Project on PyPI: https://pypi.org/project/streusle/
Currently deployment is done manually by

python setup.py bdist_wheel
python setup.py sdist
twine upload dist/*

With this change, Travis CI will deploy automatically when a tag is created on GitHub.
Also added some very basic tests for CI and a PyPI badge to the README.

[nschneid]

Thanks!