Documentation using sphinx

In this branch I've set up the basic structure for documentation using [sphinx](http://sphinx-doc.org/index.html). Sphinx is a tool for creating nice documentation. It's primarily used for documenting Python projects but there is doxygen integration as well.

I think sphinx would be a good choice as documentation system, because we would be able to provide nice documentation fast. Another advantage is readthedocs.org, which provides automatic building/hosting of sphinx documentation. The documentation of this branch is hosted at[https://projecttox.readthedocs.org/en/latest/](https://projecttox.readthedocs.org/en/latest/).

But sphinx does only support ReStructuredText, although it's possible to convert markdown files to ReStructuredText using pandoc.

This branch is just a quick proof of concept at the moment to show what's possible with sphinx.

What do you think?