Rename Prediction to TidePrediction

In order to add a SurgePrediction.

Originally I planned to put `surge_level` as a field on the `Prediction`
model but I now believe separate models is better.

https://www.pivotaltracker.com/story/show/75795364 [#75795364]
Merge pull request #81 from sealevelresearch/rename-prediction-to-tide-prediction

Rename Prediction to TidePrediction