Move SHUTDOWN_TIMEOUT to a method
Move SHUTDOWN_TIMEOUT to a method

Remove a constant per the request in #159.  Replace with a class method
which returns the same default value.

kenichi/celluloid,TiagoCardoso1983/celluloid,jstoja/celluloid,dilumn/celluloid,mehta/celluloid,olleolleolle/celluloid,sideci-sample/sideci-sample-celluloid,celluloid/celluloid,seuros/celluloid,marshall-lee/celluloid,davydovanton/celluloid,jasonm23/celluloid