By Oliver Holmes and Ned Parker BAGHDAD (Reuters) - Islamic State militants fighting in Iraq and Syria released a video on Saturday which purported to show the beheading of British aid worker David Haines. Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year while working for the French agency ACTED. The video entitled, "A Message to the Allies of America," opened with UK Prime Minister David Cameron talking about working with the Iraqi government and allied Kurdish Peshmerga forces to defeat Islamic State.