[Simple-support] add forward references

[Nuno F. <nfa...@is...>]

Hi,

Would it be possible for Simple to support forward references, such as:

<model>
    <object1 id="obj1">
        <x>4</x>
        <y>3</y>
    </object1>
    <object2 reference="obj1" />
    <object3 reference="obj4" />
    <object4 id="obj4">
        <x>2</x>
        <y>1</y>
    </object4>
</model>

Using CyclicStrategy Simple allows that object2 references object1; 
however it's not currently possible for object3 to reference object4.
Could this feature be implemented? Instead of firing the exception when 
the reference is not found, Simple could parse the all of the XML and 
then resolve references; then, if an invalid reference is found, the 
exception would be thrown.

Thank you,

Nuno Fachada
ISR - IST - Portugal