Fix a bug of backward retrieval of favorites timeline.
Thanks to the anonymous person who reported the problem.

* twittering-mode.el: Fix a bug of backward retrieval of
favorites timeline. The functions, `twittering-goto-next-status'
and `twittering-goto-previous-status', did nothing for favorites
timeline. They now treat favorites timeline as they treat other
timelines. Thanks to the anonymous person who reported the
problem.
(twittering-goto-next-status): Remove a special action for
favorites timeline.
(twittering-goto-previous-status): likewise.
Update 'README.markdown'.

* README.markdown: Add explanations of major configurations and
others.