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)

I have written a tonne of tests that compare against A&B's original C++.
Until I can distribute the `anders_briegel` package which implements
that original code, we now skip those tests if we see an ImportError.

TODO: seems a little over-zealous right now ...

Still need to "test the tests", but it's looking good.