Export `statusCode`

[luntain]

It is common to need statusCode in the code making use of this library, therefore it would be convenient to make it available together with responseStatus function. Otherwise one needs to import it from Network.HTTP.Types.Status and add http-types package to the cabal project, not to mention identifying those details in the documentation, which is rather tedious.

[snoyberg]

I'm open to a PR with these changes.