PSY- the pop singer behind the hit "Gangnam Style" -- says he "will forever be sorry" for performing a song in 2004 that called for the killing of "Yankees" ... and is begging for Americans to accept his apology. The song in question was called, "Dear Americans" ... and featured the lyrics, "Kill those f*cking Yankees who have been torturing Iraqi captives." He continues, "Kill those f---ing Yankees who ordered them to torture ... Kill their daughters, mothers, daughters-in-law and fathers ... Kill them all slowly and painfully.” Now, PSY has released a statement explaining, "The song I featured on...