BF: allow to store objects with None state, e.g. python's sets

Also adding basic test for storing sets, and few other collections
BF: import externals not only in __debug__
BF: make test_array_collectable_unique work correctly on both python2 and python3
ENH: better from_any in volume_mask_dict
Merge branch 'master' of https://github.com/PyMVPA/PyMVPA