adding lock to state when state is restored.
added conditional import of exact_cover_cxx; minor changes to work with exact_cover_cxx
adding exact_cover_cxx.py file
putting num_solutions increment back where it was to begin with ( and works )
removing state save on interrupt - overwrites correctly saved restart state for cxx version; removing decrement of num_solutions when saving complete solution for restart
adding full_solution() as required by report_search_state in puzzler/__init__.py
Merge branch 'integrate_exact_cover_cxx' into merge_lock_fix_and_exactcover_cxx