Add Django-Xadmin plugin adminx.

Add [Django-Xadmin](https://github.com/sshwsfc/django-xadmin) plugin adminx. if ``Model`` is ``MPTTModel`` subclass, admin will perform mptt effect automatic. 

Use xadmin ::

    import xadmin
    from models import Genre
    
    class GenreAdmin(object):
        list_display = ('name', 'parent', )
        list_display_links = ('name',)
        search_fields = ['name']
    
    xadmin.site.register(Genre, GenreAdmin)