Export Resolver constructor

I propose that the Resolver constructor is exported, or that a separate constructor function is exposed.

As it is right now it is impossible to create a custom Resolver, with a function of type Name -> IO (Maybe Address).  To create a custom
dynamic resolver, the Resolver constructor needs to be exported:

```{haskell}
nodes <- newMVar []
let resolver = Resolver (\name -> readMVar nodes >>= return . lookup name)
```

would create a dynamic resolver that could be altered at runtime.