Use a Router instead of an Application.

This switches to using a Router instead of a brand new Application. The reason for this change is that Routers use their host app's options, whereas a child app has its own set of options (which are the defaults).

This caused a problem for me in an app I'm building that uses application-wide options to use Handlebars. 

```js
app.set('view engine', '.hbs');
app.set('views', VIEWS_DIR);
app.engine('.hbs', exphbs(hbsOptions));
```

These options worked fine until I switched to node-express-routebuilder. After making this change, though, it worked.

Another option would be to allow routeBuilder to accept options to pass to the app, but I prefer this solution.