Fix test runner for Python 2 and 3

Hopefully for the last time!
Merge pull request #32 from sealevelresearch/fix-test-runner-again

Fix test runner for Python 2 and 3
Add date input to Tide Windows calculator

Rather than using the tide windows `now/` endpoint, you can now select a
date. This will search for tide windows for the 24 hour period starting
midnight on that date.

The dateString state is now propagated down into TideWindowsCalculator as in
TideLevelsCalculator.

https://www.pivotaltracker.com/story/show/75794694
[Delivers #75794694]