[core] Fix docstrings related to issue #125

The fixes follow the
[PEP8](http://www.python.org/dev/peps/pep-0008/#documentation-strings)
and therefore the [PEP0257](http://www.python.org/dev/peps/pep-0257/).

Please excuse my english if some mistakes can be found in my corrections. I am not an english native speaker. Therefore some sentences might look weird.

This commit only concerns the docstrings of the willie's core files (i.e. the module files are not concerned here).