Set default cookie path to app's base path instead of "/".

This allows running multiple apps in subfolders under document root, without
interfering with each other or potentially other non-cake apps on same domain.
@ADmad sorry, should have been clearer; meant to make `Session` (and `Cookie`) default `$conf['cookiePath']` to `/` if `cookiePath` is empty, then those ternary operators wouldn't be required; Wel could just pass `$config['base']` and have Session/Cookie set it to `/` if it's empty.

Not a big issue, but a bit cleaner and less DRY