defaultBundle is not longer required, but optional

When enabling the express-yui's view implementation, we used to do:

```
app.set('view', app.yui.view({
    defaultBundle: '<app-pkg-name>',
    defaultLayout: 'index'
}));
```

From now on, if the app itself is the one providing the templates to be compiled, there is not need to define it since `express-yui` is now smart enough to figure that one. And this will be just enough:

```
app.set('view', app.yui.view({
    defaultLayout: 'index'
}));
```