(whitespace-version): Update to 2.6

(whitespace-version): Update to 2.6
(whitespace-display-in-modeline): Add custom
variable to control displaying the whitespace errors on the
modeline based on suggestion from <klaus.berndl@sdm.de>
(whitespace-buffer): Now returns `t' if unclean
and `nil' if clean, to enable using as a hook function, as
suggested by Puneet Goel <puneet@computer.org>
(whitespace-buffer): Fix a subtle bug that cleaned
up only if quiet was not set, even with whitespace-auto-cleanup set.
(whitespace-mode-line): Now uses less modeline
real estate and shows whitespaces as W:<x>!<y> where <x> is any
type of whitespace which is present in the file, and <y> is any
type of whitespace whose check has been suppressed by setting
`whitespace-check-<whitespace-type>' to nil.
(whitespace-unchecked-whitespaces): New function
to return the list of whitespaces for whom checks have been
suppressed.
(whitespace-display-unchecked-whitespaces):
Renamed to `whitespace-update-modeline' to reflect its functionality.
(whitespace-version): Update to 2.5