Add support for public configuration

I've included some samples in the tests dir.  This goes against the grain with what I've seen in with the rest of the tests, where test fixtures are generated and destroyed.  IMO it is much easier to understand how configurator works when there are tangible examples.

As for the implementation, I decided to go with a file called `public-data.json` in app config.  This information is totally different and unlike the other config so I thought it appropriate if the file unlike the others.

ping @stefanor

closes https://github.com/yola/yoconfigurator/issues/28


Since public config does not need any input, I thought this was a simpler approach.  Yet for the sake of conformity, I'm not opposed to a function.

Yay/nay something like:
```python
def public_data():
    """The subset of configuration keys to be made public."""
    return [
        "myapp.hello",
        "myapp.some.deeply.nested.value",
        "myapp.oz"
    ]
```