[XMLSCHEMA-30] This commit fixes the XMLSCHEMA-30 bug.

equals/hashCode methods pairs have been added in following classes:
     - XmlSchemaNamedImpl
     - XmlSchemaNamedWithFormImpl
     - XmlSchemaType
     - XmlSchemaElement
     - XmlSchemaGroup

 NamedItemsEqualityTest unit-test has been created.