Add support for xs:extension

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:xxs="http://something.com/schemas/xtension/"
           targetNamespace="http://something.com/schemas/xtension/">

<xs:complexType name="animalinfo">
  <xs:sequence>
    <xs:element name="race" type="xs:string"/>
  </xs:sequence>
</xs:complexType>


<xs:complexType name="personinfo">
  <xs:complexContent>
    <xs:extension base="xxs:animalinfo">
    <xs:sequence>
      <xs:element name="firstname" type="xs:string"/>
      <xs:element name="lastname" type="xs:string"/>
    </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

<xs:element name="simpleemployee" type="xxs:personinfo"/>

<xs:complexType name="fullpersoninfo">
  <xs:complexContent>
    <xs:extension base="xxs:personinfo">
      <xs:sequence>
        <xs:element name="address" type="xs:string"/>
        <xs:element name="city" type="xs:string"/>
        <xs:element name="country" type="xs:string"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

<xs:element name="employee" type="xxs:fullpersoninfo"/>

</xs:schema>