Format dataset as data package

Format the data as a [data package](http://www.dataprotocols.org/en/latest/data-packages.html). The structure of the repository was already ok, but a `datapackage.json` has been added, describing the dataset. The documentation lacked some specific info, so I have used examples [here](https://github.com/datasets/gdp/blob/master/datapackage.json) and [here](http://data.okfn.org/standards/data-package) to complete the file.

All fields are now described and the license is indicated.

Currently, the repository also adheres to the more strict [Simple Data Format](http://www.dataprotocols.org/en/latest/simple-data-format.html), even with the README and script file, which are allowed. But since I plan to upload a `geojson` version of the data as well, the data package will no longer fit the SDF requirement that all data are provided as csv.