Added nbsphinx (https://nbsphinx.readthedocs.io/en/0.4.2)

This sphinx extension makes it easy to add ipython notebook directly to sphinx documentation. A notebook can be simply added to a toctree and will be automatically translated to rst (html). The packages need ipython. I also added the the sphinx rtd theme - this seems to be used also in 'testtools' but was not installed through setup.py. I also added a bibtex extension for sphinx which allows using standard bib files in sphinx rst files.