data/ml/answer-train.py: New data already has log-score for relevance, passage
AnswerFV: Store labels in the TSV file, display with weights in answer-train.py
data/ml/answer-train.py: Print # of answersets, answers
data/ml/answer-train.py: Remove LogisticRegression intercept_scaling parameter, no contribution
data/ml/answer-train.py: Use prec70 instead of cfier_any_picked as model selection criterion
AnswerScoreLogistic: New method for scoring answers based on logistic regression

It seems performance might still be worse than AnswerScoreSimple, but we
need integrating more features now, so let's try it anyway.