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 before you take any action on this comment, let me complete this review. I think I have a global comment that makes more sense.