R3931 - Register exit(1) as a shutdown function, to allow Kohana shutdown properly

Originally submitted by @zombor in [R3931](http://dev.kohanaframework.org/issues/3931) that `Kohana_Exception::handler` should `exit(1)`.

Also related to [R4794](http://dev.kohanaframework.org/issues/4794) and to its duplicate [R4275](http://dev.kohanaframework.org/issues/4275) in version 3.2, however these are not affected in version 3.3 because of the availability and use of `Kohana_Exception::_handler` (with underscore).

Through a shutdown function, we will postpone the `exit(1)` after Kohana shutdowns properly, i.e. after `Kohana::shutdown_handler`.

Ref #547. Please review and merge. Thanks!