CHANGES:
- Implemented margin-based active learning.
- Training methods now receive development data.
- Made extract_features to
  (a) Also return sentence-position indices of extracted instances.
  (b) Extract features from all (labeled or unlabeled) instances if specified to. This is used for predicting raw observations.
  (c) Optionally skips extracting features from specified instances. This is used for active learning.
- Added an option to just analyze the given data file.
- Brought back clear_spelling_feature_cache.
- Added __str__ method for SequenceData.