Begin work on sequence labeling component.
Add observer function for whether the observation is tagged.
Add function for reading PTB pos tagged files into labeled sequences.
Fix bug where the regex was unused in extract_sequences.
Merge branch 'develop' into sequence
Merge branch 'develop' into sequence
Merge branch 'develop' into sequence
Merge branch 'develop' into sequence
Minor work on feature generation for sequence data for POS tagger.
Merge branch 'sequence' of github.com:meta-toolkit/meta into sequence
More work on CRF implementation.

Copied over the trellis implementations from previous HMM implementation.
I will likely port that too...
Initial CRF model implementation.

This is very slow because I'm generating features for things that don't
appear in the training data. Next step is to adopt what CRFsuite does by
default: only making features for things observed in the training data.
More work on CRF implementation. Something's broken...
Continue working on CRF implementation; still buggy.
Fix the last(?) nasty bug in the CRF training.
Add learning rate calibration to crf training.
Make trellis a single array.
Remove unneeded computation in calibration.
Update convergence testing to better match what crfsuite does.
Ensure that the transition marginals table is initialized.