Add tests for tide-windows/now/ endpoint

- use new freezegun library to mock the time in tests
Implemented `tide-windows/now` endpoint

Now you can visit ie
/predictions/tide-windows/liverpool-gladstone-dock/now/

https://www.pivotaltracker.com/story/show/74308896

[Delivers #74308896]
Merge pull request #31 from sealevelresearch/add-now-endpoint

Implemented `tide-windows/now` endpoint