Support added and removed derivation model on a ComplexContent Diff

[ElWoodyl]

Hello,

This fixes a NullPointerException occuring when the derivation.model of a or b is null. This issue happens when trying to compare two xsd, if you have a complexContent that has a change like this one:

Before

<xs:complexType name="Model">
    <xs:complexContent>
      <xs:extension base="tns:ModelBase">
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

After

<xs:complexType name="Model">
    <xs:complexContent>
      <xs:extension base="tns:ModelBase">
        <xs:sequence>
          <xs:element type="tns:ElementType" name="newElement"/>
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

Related issue : https://groups.google.com/forum/#!topic/soa-model/nC-xO2O5m5o