dir-locals.el: Adds White Space support

dir-locals.el: Adds White Space support

Trailing white spaces will be now always highlighted, not just in
prog-mode.

Also, the White Space package, which is available since GNU Emacs 22,
is loaded and activated locally in prog-mode.

Additionally, using White Space variables, we set highlighting through
faces on wrong indentation and the maximum length of a coding line.

Notice that:
 - The highlighting for the characters beyond the set length of a
   coding line is not activated by default, only for wrong
   indentations.
 - If the White Space package is not available, errors on loading or
   activation are ignored.
 - If the White Space mode is not activated the set variables would
   not have any effect.

v2: Removed too long lines trail highlighting, as suggested by Ilia
    Mirkin.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>