Documentation using sphinx

This pull request contains a stub for the project documentation. The documentation can be build using:

```
python setup.py build_sphinx
```

Documentation for members is done via docstrings for the bindings (see docstrings.hpp) and for the mixins. The class documentation is added directly to the respective sphinx/rst document (see docs/source/api_data.rst). 

The documentation should be complete for Block. Please build the doc after merging this pull an give some feedback.

When working on nixpy please add the docstrings and class documentation to the binding you are working on.