Json-flatten

- Added documentation for the flattened format. [9](https://github.com/simonw/json-flatten/issues/9)

- `flatten()` function now verifies that the object passed to it is a dictionary, raises `TypeError` otherwise. [5](https://github.com/simonw/json-flatten/issues/5)

- Changed serialization format used for array keys, so now a dictionary with keys that are strings containing only digits are handled correctly. [1](https://github.com/simonw/json-flatten/issues/1)
- Fixed bug where empty lists were not round-tripped correctly. [3](https://github.com/simonw/json-flatten/issues/3)