Cookie management using Cookie:: class.

Cookie management using Cookie:: class.
 * Cookie::set() to set a cookie for a given number of days.
 * Cookie::kill() to kill a cookie.

Both methods change the value of the cookie both remotely (cookie stored by
the browser) and locally ($_COOKIE and Cookie::).

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>