Don't require browse-url. Require thingatpt.

Don't require browse-url.  Require thingatpt.
(goto-address-fontify-p, goto-address-highlight-p)
(goto-address-url-face, goto-address-url-mouse-face)
(goto-address-mail-face, goto-address-mail-mouse-face): Doc fix.
(goto-address-url-regexp): Use thing-at-point-url-regexp.
(goto-address-fontify, goto-address-at-mouse): Simplify,
(goto-address-at-point): browse-url-url-at-point,
goto-address-find-address-at-point can return nil.
(goto-address-find-address-at-point): Return nil on failure.