Add LabelSpreading semi-supervised classifier to the list of classifiers. Add -1 (meaning unknown label) as a possible data point.
Add support for semisupervised learning to the core.
For uniformity, rename extractors parameter to classifiers.
Use precomputed evidence in extract_facts, instead of querying it again each time.
Put SVM back as the default classifier.
Merge branch '161-label-propagation' into develop