Fixes#372.
* python/spot/impl.i: Refactor the handling of exceptions using a
Lippincott function. Map out_of_range to IndexError. Add
PyObject* version for bdd_dict's register and unregister functions
so we can use Python objects as well.
* tests/python/bdddict.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the changes.
