Make docstrings much more relevant, and more!

Every function should now have a docstring, and the sphinx documentation will
show the appropriate parameters that each function takes. All function whose
sphinx parameter list contains `*args, or **kwargs` now have proper references
to functions explaining the parameters available.

Other changes:

 * Replaced class-implemented decorators RequireCaptcha and SleepAfter with
   function-implemented decorators require_captcha and sleep_after
 * get_flair returns None when the username entered does not exist
 * Deprecated get_all_comments