As world leaders gather in New York this week at the U.N. headquarters, a Michigan congressman is inviting Americans to join a social media campaign designed to draw attention to a former Marine sergeant who has been held prisoner in Iran for two years. Rep. Dan Kildee, a Democrat from Michigan, is working to boost global awareness of the imprisonment of one of his constituents. Former Marine Amir Hekmati has been held prisoner in Iran for two years. Kildee is encouraging members of Congress to have their photos taken with a sign that says “#FreeAmir” and share them on social…