TODO XML_Serializer:
--------------------
- find a way to detect references in the structure and add id/idref attributes
- options to set the attribute names for type hints
- serialize some other structures, especially XPath return values
- custom serialization of objects
- namespace support
- alternative way to define aggregates
  array(
            "foo" => array( "bar", "argh" )
        );
  serializes to:
  <foo>bar</foo>
  <foo>argh</foo>

TODO XML_Unserializer:
----------------------
- options to set the attribute names for type hints
- some way to handle attributes (convert them into arrays)
- support for xml:space attribute
- set types for certain tags
- namespace support
- custom unserialization for objects (static unserialize() or fromXML() methods)
- unserialize references using id/idref
