Add python google style docstrings.

You can see an example of the style here http://sphinxcontrib-napoleon.readthedocs.org/en/latest/example_google.html and here https://pythonhosted.org/an_example_pypi_project/sphinx.html#full-code-example
P.S. I only just started using UltiSnips, so I apologize in advance if I did something weirdly.