Confjson

* Made filenames of both user.config.json and default.config.json configurable.
* Added `use_placeholders` argument to Config class. When True, doesn't raise KeyError on attempted access to a nonexistent key, instead returning a placeholder. Assignments of keys to a placeholder will propagate up through the hierarchy and add real dicts as needed.

Added `keys`, `get` and `__contains__` to `ConfigItemProxy` to fix a few problems caused by 1.2.1.

* Enable mixed dict and attribute style access.

- Added attribute-style access to config items.

- Improved handling of nested dicts.
- Added a check to prevent insertion of JSON-incompatible values.
- Made the tests more black-boxish to facilitate upcoming changes to the package internals.