Allow authentication database option.

This is a pretty crude way to allow authentication to a different database than the one we are connecting to.
Mainly used to allow authenticate to the admin db before using any other.

    require('mongoskin').db('root:root@dev.noodles.gr/voodoo-geoah', {
      safe: true,
      authdb: 'admin'
    }).collection('settings').find().toArray(function(err, items) {
      if (err) {
        return console.error(err);
      } else {
        return console.dir(items);
      }
    });

I've been using mongoskin for couple of hours so please excuse me if I have missed something very obvious.