Add Minute ForeignKey to Prediction model

And add a management command to create new Minutes and link Predictions.

It's very slow to do this, so doing it as a management command rather
than a migration to minimise downtime on deploy.

https://www.pivotaltracker.com/story/show/76364260 [#76364260]
Merge pull request #52 from sealevelresearch/add-minutes-to-predictions

Add Minute ForeignKey to Prediction model