Add more advanced xml writer.

[d3f3kt]

This new xml writer provides 3 more functions which are essential to handle complex xml structures.

1. It's now possible to provide attributes to the root element. Because this attributes are given as an additional constructor argument, i have created a new class to avoid bc breaks.

As an alternative, a factory could be used which would call a setter Method.

2. Nested array can be handled now.
3. XML attributes can be defined with a traling '@'.

Example:

$writer = new \Port\Xml\AdvancedXmlWriter(
    new \XMLWriter(), '/tmp/aaa.xml', 'rootElement',
    ['rootAttribute' => 'rootValue']
);

$xml = [
    'key1' => 'value1',
    '@attribute1' => 'attributeValue1',
    'collection' => [
        'foo' => 'bar',
    ],
];

<?xml version="1.0"?>
<rootElement rootAttribute="rootValue">
    <item attribute1="attributeValue1">
        <key1>value1</key1>
        <collection>
            <foo>bar</foo>
        </collection>
    </item>
</rootElement>