Fixes #35
Fixes #35

Changed Universes._list and Data._list to keys() methods.

Originally I didn't want aggregators that work like key-value pairs, but
are certainly not dictionaries, to behave too much like dictionaries,
lest they be expected to work in every way like dictionaries. I'm not so
worried about this now, especially with respect to keys() methods, since
those are so commonly needed when working with key-value data
structures.

datreant/datreant.core,datreant/datreant.core,andreabedini/datreant,datreant/datreant.data,dotsdl/datreant,datreant/datreant,datreant/datreant