DeepDive 0.031

- Updated example walkthrough and `spouse_example`
- Added python utility `ddlib` for text manipulation (need exporting PYTHONPATH, usage see its pydoc)
- Added utility script `util/extractor_input_writer.py` to sample extractor inputs
- Updated nlp_extractor format (use sentence_offset, textual sentence_id)
- Cleaned up unused datastore code
- Update templates
- Bug fixes