UITextField实现左侧空出一定的边距

就是通过uitextfield的leftview来实现的，同时要设置leftviewmode。

如果设置左右边距，需要再加上rightView功能

-(void)setTextFieldLeftPadding:(UITextField *)textField forWidth:(CGFloat)leftWidth
{
    CGRect frame = textField.frame;
    frame.size.width = leftWidth;
    UIView *leftview = [[UIView alloc] initWithFrame:frame];
    textField.leftViewMode = UITextFieldViewModeAlways;
    textField.leftView = leftview;
}