NetworkX provides `add_node`, we don't, leads to missing
`graph.node["vop"]`. Does this fix it?

We needed to implement `GraphState.from_json` so that the server can
track a copy of the client's state. That's a useful byproduct of this
work!

`prefix` was used for testing against original implementation

Fixes #8, "Allow control over which `GraphState` operations are depicted by server". Enables use of multiple visualized GraphStates in the same program. Downside -- you have to call `update()` at the end of your program.

Moved that functionality into `abp.GraphState`, so that you can
just run `state = abp.GraphState(nx.Graph(((0,1),)))`, `state =
abp.fancy.GraphState(nx.Graph(((0,1),))` or whatever.

Now you can write `abp.fancy.GraphState(nx.Graph())`.
TODO: make this work for a non-fancy `GraphState`

Very functional, very explicit node names - working so far

To play around with lattices

Trying to get more sensible defaults for people who like to think about
graphstates rather than circuits.

Repsponse to issue #4: [Link](https://github.com/peteshadbolt/abp/issues/4)

Empty for new

Is this going to fix errors with different versions of networkx? I don't
know -- I'm on a plane mate.

- `GraphState.copy()`
- `measure_x` etc.
- `measure(detail=True)`
- Better documentation on `remove_vop` and `act_local_rotation`