add transparency mode for png images

This feature includes transparency mode for png files. As default transparency mode is set to false.

```php
QRcode::png(
    $text, 
    $outfile = false, 
    $level = QR_ECLEVEL_L, 
    $size = 3, 
    $margin = 4, 
    $saveandprint = false, 
    $back_color = 0xFFFFFF, 
    $fore_color = 0x000000, 
    $back_transparent = false
) 
```

Demo page `index.php` is updated to test and show transparency.