NSWorkspace *wc = [NSWorkspace sharedWorkspace];
NSString *urlString =@"http://taehoonkoo.tistory.com";
NSURL *url = [NSURL URLWithString:urlString];
참고 자료 :
http://stackoverflow.com/questions/2965615/open-url-with-safari-no-matter-what-system-browser-is-set-to