add constants instead of literals
add a XmlSchemaObject base class

In its constructor, it receives the XmlSchema instance it
belongs to and the DOMNode containing its definition, ex-
posing both as protected properties.

All classes inherit from XmlSchemaObject and
must pass the schema and the definition node
data to the parent constructor.
add XmlSchemaParticla base class and move node classes to subfolder
XmlSchema: throw exception when node cannot be found
add dummy compositor classes
begin working on complex type coercion