TODO XML_Serializer:
--------------------
- find a way to detect references in the structure and add id/idref attributes
- serialize some other structures, especially XPath return values
- serializes tags with attributes
  array(
          "_attributes" => array( "foo" => "bar" ),
          "child"       => "content"
       );
  serializes to:
  <array foo="bar">
    <child>content</child>
  </array>
- custom serialization of objects
- namespace support

TODO XML_Unserializer:
----------------------
- add support to convert arrays into objects
- support for xml:space attribute
- set types for certain tags (like in patConfiguration)
- namespace support
- custom unserialization for objects (static unserialize() or fromXML() methods)
- unserialize references using id/idref
- implement callback mechanism
