Allow tweaking of a cloned object prior to showing its admin form

This allows for twaks prior to diplaying the cloned object in its admin
form. For example, you could append " (duplicate)" to the object name.

NOTE: I added a sample use of the new feature in the sample project to advertise its use, but that had rather important consequences in the tests. Do you think it's too much? We could remove the sampleproject example, but then I don't know where we would document it. I'm a bit afraid to overcrowd the README (and we don't have full docs yet).
I believe we could find a better name for `fkname`, the value being passed is the `related_name` parameter we give to `ForeignKey`

When I first read `fkname` the first think I thought was the name of the field defined in `Comment` that is a `ForeignKey` to `Post`.

Something else to keep in mind is that if there is more than one inline to the same model, we'll add a `-2` to the name (and keep adding), like `comment_set-2` then `comment_set-3`

Anyway, can you think of clearer name for this parameter? Maybe even `related_name` would be fine. :)