Pass options object to replset

When you're using ReplSet, options object isn't passed to a `new Db()` call, which makes it impossible to set options such as writeConcern:

```javascript 
var mongode = require('mongode');

// this default writeConcern is currently ignored --v
var mongo = mongode.connect('test', '127.0.0.1', {w:1});
    mongo.collection('test').insert({foo:1}, function() {
        console.log(arguments);
    });
});
```