[WIP] Migrate packaging to Poetry

[nchammas]

Alternative to #252.

• Update Travis CI build
• Understand impact on PyInstaller distributions
  • Seems to be no impact
• Test publishing of sdist and wheel
  • Are all the same files included?
• Do we still need __main__.py?
• Update README
• Update Contributing guide

Potentially blocking issues:

• Getting flintrock on the PATH: poetry install should make console script entry points available on the current PATH python-poetry/poetry#311
• Dealing with multiple Python versions: Poetry fails when Poetry's Python version differs from the active environment's Python version python-poetry/poetry#354

Other issues:

• No clean way to single-source the application version: How to get version from pyproject.toml from python app? python-poetry/poetry#273

Fixes #248.

[nchammas]

I think this might be worth revisiting some other time. For now, I'll stick with something more conservative, like in #252.