Strtree

- Added multi-class classification, with same API
- Added StringTree.predict_label, StringTree.get_nodes_by_label methods
- Improved performance for large datasets
- Minor docs and docstrings changes

- add escaping special characters
- major changing of importing mechanism in dummy_examply.ipynb
- minor changes of docstrings
- minor adding of tests

- fixed min_matches_leaf, min_strings_leaf behavior

Initial relase.
All the major functionality is covered by tests.