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."
-- http://www.python.org/dev/peps/pep-0008/

Also fixed one of the doctests. '_link' attribute for gh followed
users does not exist, corrected to 'url'.
Merge pull request #4 from candeira/pep8_exceptions

Correct Exception inheritance according to PEP8