add documentation/ follow google style guide

[maxnoe]

We need to add more documentation, docstrings for all classes etc.

The google style guide might be helpful:
http://google-styleguide.googlecode.com/svn/trunk/pyguide.html

[ibab]

I prefer the numpy style of documentation: https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt