As mentioned in https://developer.github.com/v3/orgs/, an organization also has a description. This PR adds this attribute to the `Organization` class, as well as modifies its `edit` method so that it's possible to update it.
I noticed too late the [`add_attribute.py` script](https://github.com/PyGithub/PyGithub/blob/8ae2bcb1e6f96beeec99fbbcf00f4af46bb38cde/scripts/add_attribute.py), so I did my modifications by hand, but I noticed that the output of the script and the style used for all the other attributes are different. I preferred the style that is prevalent in the file, rather than the output of the script.
Jacopo Notarstefano
1d91880967