(whitespace-version): Update version to 2.4

(whitespace-version): Update version to 2.4

Add customization variables to conditionally test
any of the five whitespaces.

(whitespace-spacetab-regexp): Fix doc string.

(whitespace-modes): Add `change-log-mode' to the
list of modes to be checked for bogus whitespaces.

(whitespace-rescan-timer-time): Update documentation.

(whitespace-display-unchecked-whitespaces):
new function to update modeline with untested whitespaces.

(whitespace-buffer): Test only whitespaces whose
checking is turned on, and update modeline using the newly created
`whitespace-display-unchecked-whitespaces'.

(whitespace-cleanup): Cleanup only whitespaces
whose checking is turned on, and update modeline using the newly
created `whitespace-display-unchecked-whitespaces'.

(whitespace-describe): Update documentation.

(whitespace-tickle-timer): Test if `whitespace-rescan-timer-time' is
non-zero before tickling timer.