Added method to set width of a barcode on Printer

[longsview]

This method allows a user to set the element width of the barcode which is handy when printing long barcodes.

[mike42]

Thanks for your contribution. I've made some additions to the new feature, and this pull request will be merged to master when the next release is made.

The additions I made are linked to this pull request:

• Wrote test cases over the new function
• Added the new function to the README
• Updated the barcode example to show how it's used
• Added you to the contributors file
• Edited the default bar width from 2 to 3, which appears to be consistent with the width of a barcode on a newly-initialised printer

To illustrate, the example snippet is:

/* Height and width */
$printer->selectPrintMode(Printer::MODE_DOUBLE_HEIGHT | Printer::MODE_DOUBLE_WIDTH);
$printer->text("Height and bar width\n");
$printer->selectPrintMode();
$heights = array(1, 2, 4, 8, 16, 32);
$widths = array(1, 2, 3, 4, 5, 6, 7, 8);
$printer -> text("Default look\n");
$printer->barcode("ABC", Printer::BARCODE_CODE39);

foreach($heights as $height) {
    $printer -> text("\nHeight $height\n");
    $printer->setBarcodeHeight($height);
    $printer->barcode("ABC", Printer::BARCODE_CODE39);
}
foreach($widths as $width) {
    $printer -> text("\nWidth $width\n");
    $printer->setBarcodeWidth($width);
    $printer->barcode("ABC", Printer::BARCODE_CODE39);
}
$printer->feed();

Which presents this output: