FeatureVector: Expand some asserts with messages to help tell what's going wrong
FeatureVector: First implementation of label-attached features (for classifier)
Classify/FeatureVector: Allow for label-attached features, redo FeatureVector factories
UserFrequency: Need to cast to long before shifting, even if right operand is long
FeatureVector/Ranker: Merge unigram/ngram features
Reranker/Cleanup: Fix comment in PointwiseClassifyingRerankerTrainer
New/ClassifierRanker: Add classifier as possible ranker