Zeitgeist Stage Company is rolling through a zinger of a play with "Valhalla," the wit-drenched Paul Rudnick play underway within the Boston Center for the Arts. Two hours have passed by quickly, as Zeitgeist's six-member cast has deftly brought more than 20 laughter-inducing characters from two time periods, to a simple and adaptable stage. The work is frothy fun, but then Rudnick's work throws the company a curve ball. Were "Valhalla" to end 10 minutes......