Hgijson

Changed
- Fix for issue with fix in `1.4.1`.

Changed
- Fixed bug in serialisation of `None`,
[reported](https://github.com/wtsi-hgi/python-json/issues/16issuecomment-237527405) by
[YuriIvanov](https://github.com/YuriIvanov) ([18](https://github.com/wtsi-hgi/python-json/issues/18)).

Changed
- Encoders now serialise `None` to `null` and visa-versa for decoders (thanks to
[YuriIvanov](https://github.com/YuriIvanov) for bringing this improvement to light in
[16](https://github.com/wtsi-hgi/python-json/issues/16)) ([17](https://github.com/wtsi-hgi/python-json/issues/17)).
- Corrected self-referential type hinting.

Changed
- Corrections to `setup.py`.

Added
- Documentation on how to serialize to/from a dictionary opposed to a string.
- Documentation on how to install the library, import methods/classes and develop.
- imports via `__init__.py`.

Changed
- Removed useless equality and hash calculation methods on `PropertyMapping`.
- Improvements to documentation.

Removed
- Helper test runner scripts.

Changed
- Hotfix for dependency installation when installed via PyPI.