Standardized project structure

Privet Alex,

I've added a standard `setup.py` file which allows users to install the widget via `pip`, for example:
```
git+http://github.com/egasimus/django-split-json-widget.git@master#egg=django-split-json-widget
```
This way, the widget can also be added as a dependency in a project's `requirements.txt` for automated installation of the latest version. I also cleaned up `README.md` a bit, and a few extra improvements might be on the way.

Keep up the good work!