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
Merging in master updates

Conflicts:
	nengo/__init__.py
	nengo/builder.py
	nengo/networks/basalganglia.py
	nengo/networks/circularconvolution.py
	nengo/networks/integrator.py
	nengo/networks/oscillator.py
	nengo/objects.py
	nengo/tests/test_circularconv.py
	nengo/tests/test_ensemble.py
	nengo/tests/test_integrator.py
	nengo/tests/test_networks_basalganglia.py
	nengo/tests/test_node.py
	nengo/tests/test_oscillator.py
	nengo/tests/test_simulator.py
Fixed some errors introduced by merge
Cleaning up unused imports