Get a very basic test of API working

This PR does some plumbing required to get tests of the API up and running and tests a simple prediction and no-parameter error condition. It makes heavy use of unittest.mock (and the Python 2 back port) to mock out enough of the tawhiri and ruaumoko dataset API to let the web logic proceed.

It makes no modification to any tawhiri code, it just adds two tests.

The mocking is a little ugly and probably reflects the need for a little light re-factoring of the api code but I wanted to get a test working which didn't modify api.py.