Section: dict-like section creation

[gicmo]

On the mission to make working with metadata as close to dicts (and pythonic) as possible there is not that much missing. For the object creation, the properties side of things has been solved with pr #101. The thing that remains is creation of sections, currently done via section.create_section('name', 'type')
Following ideas came to my mind:
Create section via __setitem__ and a tuple, i.e. s['new section'] = ('type'). If people fear that this is too confusing to value creation (which is done via lists, i.e. s['prop'] = [1, 2, 3]) then maybe we could introduce a dummy class (e.g. a collections.namedtuple) with a short name (nix.S): s['new section'] = nix.S('type').
I think I personally prefer the first version, i.e. the tuple based approach.