Add invoke tasks.py file to make it easier to publish to PyPI

[thebjorn]

Basic usage:

• pip install -r requirements.txt (to get invoke and twine)
• inv -l to list available tasks
• inv version to print current version
• inv setversion 1.2.3 to set the version to 1.2.3.
• inv build --clean --upload to remove build/dist directories, build sdist and wheels, and finally use twine to upload to PyPI.