Allow serializers to non ovewrite fixtures objects if they are marked as non overridable

[Manuel Saelices]

Look at this use case:

1. The developer creates a new Django project (i.e. with south support).
2. The developer creates the database schema and load the initial data (including in example a demo home page).
3. The manager modify home page, and other contents.
4. The webmaster upgrade the project, and executing migrate command (because south support).
5. With default Django serializers, all manager changes that exists as fixtures data will be lost.

I know ​Django docs prevent about that, but It was wonderful you may mark objects as no rewritable, like this:

!xml
<?xml version="1.0" encoding="utf-8"?>
<django-objects version="1.0">
  <object pk="1" model="fooapp.foomodel" overwrite="no">
    <field type="SlugField" name="slug">welcome</field>
    ...
  </object>
</django-objects>

Look at ​this implementation as implementation reference.