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.
@scottpurdy Ok, I'll quibble, although this is a trivial issue and the PR is already merged. I appreciate you citing the sources from PEP 8, but I looked them up too before I stated that I didn't understand, and I still don't. It seems clear to me which of the following is better:

```
_VERBOSITY = 0         # how chatty the demo should be
_SEED = 1956             # the random seed used throughout
```

```
_VERBOSITY = 0         # how chatty the demo should be
_SEED = 1956           # the random seed used throughout
```

PEP-8 doesn't say to intentionally misalign comments. The first line has 9 spaces between code and comment; the second line, 11. Is there a reason for that?

I would even prefer this over the misaligned comments:

```
# how chatty the demo should be
_VERBOSITY = 0         
# the random seed used throughout
_SEED = 1956             
```

@scottpurdy said:
> Also in PEP 8 it says not to use whitespace to align operators and I think this is good justification for not aligning comments for different statements

I disagree. Operators are code, so I understand why PEP-8 says introducing extra whitespace into a language where whitespace is syntactically important is a bad idea. But comments are not code, and readability and human understanding should be paramount when using comments. Comments are for humans.