Added Default image and border properties

Usage:

self.imageView = [[AsyncImageView alloc] initWithFrame:CGRectMake(0,
0, 200, 200)];
self.imageView.contentMode = UIViewContentModeScaleAspectFill;
self.imageView.clipsToBounds = YES;
self.imageView.borderWidth = 2;
self.imageView.borderColor = [[UIColor blackColor] CGColor];
self.imageView.cornerRadius = 7;
self.imageView.defaultImage = [UIImage
imageNamed:@"placeholder-no-image"];

[self.photoImageView addSubview:self.imageView];

self.imageView.imageURL = [NSURL
URLWithString:@"http://host.com/image.png"];