Moved PythonFunction to builder

- We no longer import it into nengo namespace (not user facing)
- Updated tests
Removed PythonFunction class

- It wasn't really necessary; the resulting code without it
  is pretty much the same, if not a bit simpler.
Rename nonlinearities to neurons

While they aren't all necessarily neuron types,
they will essentially be treated like neurons in that
they can act as Ensemble.neurons.