Support negation for the contain_exactly matcher

As far as I can tell no one just ever implemented the failure message for the `contain_exactly` matcher, fixes #625.
None of our other failure messages are worded this way.  This should be in the typical `"expected #{actual} to #{description}..."` form.  Maybe this?

``` ruby
def failure_message_when_negated
  "expected #{actual.inspect} not to contain exactly#{to_sentence(surface_descriptions_in expected)}"
end
```