Make the diff service configurable with extension points

The DocumentDiffService should expose extension points to

Configure XMLUnit for the diff, what is currently hard-coded:

protected void configureXMLUnit() {
  XMLUnit.setIgnoreWhitespace(true);
  XMLUnit.setIgnoreDiffBetweenTextAndCDATA(true);
}

Configure the diff itself, what is currently hard-coded:

protected void configureDiff(Diff diff) {
  diff.overrideDifferenceListener(new IgnoreStructuralDifferenceListener());   
  diff.overrideElementQualifier(new ElementNameAndAttributeQualifier());
}

Contribute a "second-level" differ for a given type of property.

The first-level diff detects the document properties that are different between the 2 docs, for example dc:title.
For now it also holds the different values on both sides: left and right, for example: {{

}}.

The second-level diff would use the type held by the first-level diff of a property to call the specific differ for this type on the document's property.
This implies a refactoring of the PropertyDiff implementations. The first-level diff no more needs to hold the different values, but the type of diff and a reference to the property itself (xpath like).
For example a structure like {{

}}.

This way to access the second-level diff for a property, the system would just have to call the appropriate differ contributed for the diff type. If no differ is associated to the diff type, a fallback is done on the default one.

Example of a contribution of a complexList differ:

<propertyDiffer type="complexList" class="ComplexListPropertyDiffer"/>