Global Inhibition

(Pretty picture in #56)

There's a new Edge build function in the builder which notices `Ensemble -> Neurons` connections.  As we only allow connections of this form if they're global inhibition connections (i.e., `transform=[[k]]*n`) it attempts to create a new global inhibition edge.

When filters are put together for vertices any `GlobalInhibitionConnection`s are ignored.

This'll need cleaning up when the builder refactor is done.
@neworderofjamie - this appears to be how I got around shape mismatches when doing global inhibition, it might be worth using this approach (means that we know the connections can be passed safely to Nengo builder utils to get things like full transforms).