Refs #339 Readonly fields get displayed incorrectly in History view

Instead of passing the Version parameters as initial, override them in the object itself
The initial version of the code was like that, but it raises a ValueError on foreign keys, since it is expecting an instance of the type instead of the ID. So for example if we have
```
class Book(models.Model):
    author = models.ForeignKey(Author)
    title = models.TextField()
```
the keys returned by ```self.get_revision_form_data(request, obj, version)``` are (author, title), but we need to set (author_id, title).

Also I replaced iteritems with items, since it broke the Travis build on python 3.x. Best would be to use six, but I didn't want to add another dependency.