Removed PassthroughNode

- You can make what used to be a PassthroughNode by
  making a node with output=None (which is the default)
- Changes to builder code reduced duplication, so there doesn't
  seem to be a downside to this change (he said hopefully)
Imported core API into nengo namespace

- This includes objects, nonlinearities, simulator, model,
  and networks (which stay in the networks namespace)
- Addresses issue #206
Removed old API

- Not necessarily permanently, but we haven't been maintaining it,
  so best to remove it for now.
- Addresses #208