Part of issue #376, reported by Hashim Ali.

* python/spot/impl.i: Add bindings for kripke_graph.
* python/spot/__init__.py (automaton): Add a want_kripke option.
* spot/kripke/kripkegraph.hh: Honnor the "state-names" property
when displaying states.
* spot/twaalgos/hoa.cc: Preserve names of Kripke states.
* tests/python/ltsmin-dve.ipynb: Illustrate all the above.
* NEWS: Mention those changes.
* THANKS: Add Hashim.