This merge updates abpserver to support a full set of operations
including creation / deletion, gates, local complementation, and
measurements through the web interface. So now you can click on qubits
and measure them in X or whatever.

I've tested it but I bet there are bugs -- please raise an issue if you
find one.

Conflicts:
	examples/visualization/grid_2d.py

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.