Added stub DefaultHandler class.
Implemented XmlSerialzser.handler() methods.
Added associated unit test method.
Changed DefaultHandler to be bound to an XmlSerializer instance instead of being a singleton.
Added unit test method to check if default handler is handled correctly.
Implemented XmlSerializer.serialize() methods.
Added associated unit test methods.
Added appropriate @Target and @Retention annotations to @Serializable and @Serialize.
Fixed XmlNode.newFromXml() method (changed to static).
Fixed typo in XmlNode.isValidIdent() which cause unit test to fail.
Changed type for XmlSerializer.defaulthandler to TypeHandler.
Implemented XmlSerializer.deserialize() methods.
Added associated unit test methods.
Added XmlSerializerTest to test suite.