Related to issue #376.
* spot/kripke/kripkegraph.hh: Avoid indirect type definitions for the
benefit of Swig.
* python/spot/impl.i: Add bindings for iterators over kripke_graph
states and edges.
* tests/python/kripke.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Update.
