Tens of thousands of comic book fans from around the world will swarm the Boston Convention & Exhibition Center this weekend for Boston Comic Con, a massive annual gathering that offers die-hards sneak peeks at upcoming features and opportunities to meet their favorite celebrities.The convention, which runs through tomorrow, is known worldwide for visitors’ wild, comic-themed costumes and will feature more than 100 events ranging from speed-dating and sing-alongs to celebrity meet-and-greets and autograph, photo and question-and-answer sessions.