从plist中读取 对象
NSString *path = [[NSBundle mainBundle] pathForResource:@"Presidents_a" ofType:@"plist"];
NSData *data;
NSKeyedUnarchiver *unarchiver;
data = [[NSData alloc] initWithContentsOfFile: path];
unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData: data];
NSMutableArray *array = [unarchiver decodeObjectForKey: @"Presidents"];
self.list = array;
[unarchiver finishDecoding];
[unarchiver release];
[data release];
plist