Generate namespaced element on XmlList entries 

Given this class:
```php
/**
 * @XmlRoot("root", namespace="http://example.com/namespace")
 * @XmlNamespace(uri="http://example.com/namespace")
 * @XmlNamespace(uri="http://example.com/namespace2", prefix="x")
 */
class ObjectWithNamespacesAndList {
    /**
     * @Type("string")
     * @XmlElement(namespace="http://example.com/namespace")
     */
    private $name;
    /**
     * @XmlElement(namespace="http://example.com/namespace2")
     * @XmlList(inline = false, entry = "phone")
     */
    private $phones;
}
```

The current generated  XML  is 
```xml
<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="http://example.com/namespace" xmlns:x="http://example.com/namespace2">
  <ns-9d6dff63:name xmlns:ns-9d6dff63="http://example.com/namespace"><![CDATA[myname]]></ns-9d6dff63:name>
  <x:phones>
    <phone><![CDATA[000]]></phone>
    <phone><![CDATA[111]]></phone>
  </x:phones>
</root>
```
**Current behavior problems**
* Since `root` and `name` have the same namespace, there is no need to generate a prefix.
* There is no way to decide a namespace for `phone` node.

With this PR, the generated  XML  is:


```xml
<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="http://example.com/namespace" xmlns:x="http://example.com/namespace2">
  <name><![CDATA[myname]]></name>
  <x:phones>
    <x:phone><![CDATA[000]]></x:phone>
    <x:phone><![CDATA[111]]></x:phone>
  </x:phones>
</root>
```
`phone` inherits the namespace form `phones` node. Would be useful allow a different namespace?