Implement ConfigItem via getattr, setattr overrides

@dguest, to avoid mucking about with ConfigItem.__dict__, I went even more gnarly. See just the last commit.

It's mostly as an exercise because I could believe this is more screwed up than I am aware of...

There are 3 elements:

1. We want __getattr__ to access self._data for all _data's attributes, but we have to avoid infinite recursion when retrieving _data.
2. We want __setattr__ to permit setting _data once only, so _initialised is a bit of a fake attribute (also not visible from keys()).
3. We need to override deepcopy so that it doesn't hit the _initialised block on setting _data.