Add Pipfile to support Pipenv

The problems that Pipenv seeks to solve are multi-faceted:

You no longer need to use pip and virtualenv separately. They work together.
Managing a requirements.txt file can be problematic, so Pipenv uses Pipfile and Pipfile.lock to separate abstract dependency declarations from the last tested combination.
Hashes are used everywhere, always. Security. Automatically expose security vulnerabilities.
Strongly encourage the use of the latest versions of dependencies to minimize security risks arising from outdated components.