add user-agent and login to available code samples

[fhocutt]

Improve the available code samples by demonstrating use of the user-agent function and including a sample that demonstrates use of login/cookies to make edits to a wiki.

[lahwaacz]

The usage of user agent, cookies and login is clear from the docs, I don't see what is missing here. As for the edits, you'll need to handle tokens, which brings us to #17.

[brainwane]

It's great that the docs mention abstractly how to use user agent and login/cookies. Many new users of a library benefit from also seeing concrete examples -- see http://www.crummy.com/software/BeautifulSoup/bs4/doc/ for how one library does that. Maybe label this issue "enhancement"?