Replace the surface content with the new one

I have tested in some portals and when i clicked at a link, the new content was added below of Surface (not replaced it).

Now, works fine.

Scenario:
1. Go to a portal with no senna.js (http://portalpadrao.plone.org.br/)
2. Load senna.js from DevTools console.
3. run the script:
```javascript
var app = new senna.App();
app.setBasePath('/');
app.addSurfaces('portal-column-content');
app.addRoutes([
  new senna.Route('assuntos/editoria-a', senna.HtmlScreen),
  new senna.Route('servicos/perguntas-frequentes', senna.HtmlScreen)
]);
```
Click at "Perguntas Frequentes", for example. The new page content will be loaded at bottom of page.