Add sphinx Documentation

[gregorpirgie]

Currently, there is no documentation included in source code, only the readme.

This relates to #18

Can we implement a basic sphinx documentation for bibsonomy, once the docstrings are added?

[gregorpirgie]

Before doing so, we might decide on #23 (if we want to rename the tool). Or at least keep this in mind while writing the documentation (linking the appname in the docs instead of writing it as a plain string whenever it occurs in the docs).