Add Get/Post/Env/Cookie::t() that returns a trimmed string.

Add Get/Post/Env/Cookie::t() that returns a trimmed string.
Add Get/Post/Env/Cookie::blank() that returns true if the value associated
to the given key is empty (or, contains only whitespaces).

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