Record training scores in DB

This PR adds a new column to the database to save the score on the training sets during CV. Unfortunately, I couldn't come up with a 'good' way to do this, because sklearn's GridSearchCV (which we were using) does not support returning this data. So I had to copy a lot of code from https://github.com/scikit-learn/scikit-learn/blob/0.15.2/sklearn/grid_search.py#L340.