When Walt Disney’s daughters begged him to make a movie of their favorite book, P.L. Travers’ “Mary Poppins,” he made them a promise – one that he didn’t realize would take 20 years to keep. In his quest to obtain the rights, Walt faced the curmudgeonly, uncompromising writer who had absolutely no intention of letting her beloved magical nanny get mauled by the Hollywood machine. But, as the books stopped selling and money grew short, Travers reluctantly agreed to go to Los Angeles to hear Disney’s plans for the adaptation. For two short weeks in 1961, Walt Disney pulled out all the stops. Armed with imaginative storyboards and chirpy songs…