ios修改textField的placeholder的字体颜色、

• 利用KVC修改

textField.placeholder = @"username is in here!";  
[textField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];  
[textField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];

• 搜索时，发现这篇文章写得很全，可以参考：
  http://www.jianshu.com/p/e5c6c9b455dd