Resolves #4708 - adding transparent background support to barcode

There are currently no tests for this, the code in general here could probably be cleaned up a bit.  Also, I have noticed that there is currently no 2.0 documentation on Barcode and ZendPdf doesn't exist.

Example Usage for Image:
```
        // Only the text to draw is required
        $barcodeOptions = array('text' => 'ZEND-FRAMEWORK');

        // No required options
        $rendererOptions = array('imageType'=> 'png', 'transparentBackground' => true);

        \Zend\Barcode\Barcode::factory(
            'code39', 'image', $barcodeOptions, $rendererOptions
        )->render();
```

Example Usage for SVG:
```
        // Only the text to draw is required
        $barcodeOptions = array('text' => 'ZEND-FRAMEWORK');

        // No required options
        $rendererOptions = array('transparentBackground' => true);

        \Zend\Barcode\Barcode::factory(
            'code39', 'svg', $barcodeOptions, $rendererOptions
        )->render();
```