A document that once freed slaves now hangs over the bust of a slain civil rights leader in the Oval Office occupied by the nation's first African-American President.   The Emancipation Proclamation was hung Monday, the day set aside to celebrate Martin Luther King, Jr.'s birthday, in its temporary home.  It will be on display in the Oval Office for the next six months before being moved to Lincoln's Bedroom where the original was signed by President Abraham Lincoln on January 1, 1863.  The Proclamation brought an end to slavery after the Civil War.