This one is pretty big so I suggest reviewing each commit in turn instead of the whole thing at once. This basically implements the behavior discussed here #40. In particular, there is (hopefully, I think), one way of accessing each thing. Among other things:

• H['attr'] accesses hypergraph attributes.
• H.nodes() and H.edges() are no longer supported
• H.nodes[] and H.edges[] now access attributes
• H.nodes.membership is now the same as H.nodes.members. (I decided to keep members for simplicity and symmetry with H.edges)

There are a few kinks to work out, but I thought this PR was already big enough.

As you review, please let me know if you think of any other test I should add to this PR.

Also: there's a lot of noise in this PR because I decided to change *_attrs to *_attr just to be uniform with the variable names in the main Hypergraph class.

Closes #40