This will allow users to connect to local dynamo

In order to connect to local dynamo (see #36), you just pass an additional string argument, url, to the dynasty constructor:

```javascript
var credentials = {
    accessKeyId: '<YOUR ACCESS_KEY_ID>',
    secretAccessKey: '<YOUR_SECRET_ACCESS_KEY>'
};

var dynasty = require('dynasty')(credentials, 'http://localhost:4566');
```

I would recommend setting up local dynamo using: https://github.com/Medium/local-dynamo