BF+DOC: NamedTuple is an ugly beast

Pickling has issues with Python 2.7.4 and prior.
Added test for OrderedDict too.
Merge branch 'master' of github.com:PyMVPA/PyMVPA

* 'master' of github.com:PyMVPA/PyMVPA:
  BF: workaround for ds.summary() if one of the targets is None
  DOC: minor styling