Added ClonableModelAdmin.tweak_cloned_obj()

This allows for twaks prior to diplaying the cloned object in its admin
form. For example, you could append " (duplicate)" to the object name.
Added support for inline tweaking

The previous ``tweak_cloned_obj()`` approach didn't let us tweak inline,
making the feature a lot less useful. Now, there's
``tweak_cloned_fields()`` and ``tweak_cloned_inline_fields()``.

ref #13
In tweak_cloned_inline_fields(), rename fkname to related_name

It's more descriptive. Also, added a note in the docstring about the
effect of having a relation inlined more than once.

Related to PR #13.
Fixed typo in tweak_cloned_inline_fields()

s/prefix/suffix :(

Related to PR #13.
Merge pull request #13 from savoirfairelinux/tweak-cloned-obj

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