Correct Exception inheritance according to PEP8

"Derive exceptions from Exception rather than BaseException.
Direct inheritance from BaseException is reserved for exceptions
where catching them is almost always the wrong thing to do."

Also fixed one of the doctest. No '_link' attribute for gh followed
users, corrected to 'url'.

By the way:
I love your dynamic style for this github api client. I like that I can use githubpy with the documentation from the API, with no extra library documentation once I got started. Well done and thanks.