Generate namespaced element on XmlList entries

Things done with this patch:
- Merged #237 
- `@XmlNamespace(uri="http://example.com/namespace")` acts as default namespace
- `XmlDeserializationVisitor`
 - Removed a lot of XPath stuff in flavor of simpler `SimpleXMLElement::children` method
 - Added `$objectMetadataStack` to access class metadata from a property
- `XmlSerializationVisitor`
 - Removed unnecessary element prefixing when the document/node default namespace match the desired namesapce
 - Removed some duplicated code, adding `createElement` and `setAttributeOnNode` methods



I'm not sure if an implementation as:
```php
private function getClassDefaultNamespace(ClassMetadata $metadata)
{
    return (isset($metadata->xmlNamespaces[''])?$metadata->xmlNamespaces['']:null);
}
```
would be enough...