Add DisplayCase::Exhibit#reexhibit method.

This is a shortcut for calling `my_exhibited_object.exhibit(to_model)`
... which is needed when passing an exhibited object through a `render`
call as a local variable.
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See: http://keepachangelog.com/ for rationale.
Remove DisplayCase::Exhibit#reexhibit; Do reexhibit when rendering, automatically.
fix tests after merging #51
bump version happy :birthday: to me