redirectUri is now a function

Now you can dynamically change the redirectUri by passing a function

``` js
var auth = gh(id, secret, {
  redirectUri: function(req) {
    return 'http://example.com?from='+request.headers.host;
});
```

This also means not exposing `ghUrl` from the module. You should probably also just use the `auth.login` middleware instead - I've updated the examples to reflect this.