Restructure to make installable with setuptools

Ok, so related to #20 I removed `simplejson` and added a `setup.py` specifying simplejson as a requirement. I did not add a specific version, since I do not know... 

I converted the README to reStructuredText, since it can then be used at PyPI as well. In the process I made a few changes and extracted the license into a new file `LICENSE.md`. Afterwards I realised it would perhaps have been a cleaner approach to first commit a direct conversion and then make a new commit with the changes, but I hope it's ok.