When I heard 'Zombieland's Ruben Fleischer was directing a period gangster flick, I was pumped. Honestly, however, all I really demanded of it was that it was as fun as 'Zombieland.' If this trailer is any indication, 'Gangster Squad' looks like a boatload of fun. Tommy guns blazing, one-liners flying about and swirling in our ears like spent shell casings on the floor, and an absolutely amazing…