BAINBRIDGE ISLAND, Wash. — The Rev. Dr. Martin Luther King Jr.’s speech at the March on Washington for Jobs and Freedom in 1963 was unusual among great American speeches in that its most famous words — “I have a dream” — were improvised. King had certainly thought about using the “dream” refrain in Washington. He had been fine-tuning it earlier that year. In April, in Birmingham, Ala., deputies of