Network example

Resolves numenta/nupic.core#154

Adds the example `examples/network/network_api_demo.py` that computes identical anomaly scores to `examples/opf/clients/hotgym/anomaly/hotgym_anomaly.py` but using the Network API directly instead of the CLAModel and OPF.
Ok, I'm coming from the newbie viewpoint here. But I don't understand what `py.RecordSensor` represents exactly. I can see that is a node type, but perhaps we can explain it a bit here?