allow rendr to be mounted on nested url

Backbone.history.start can take a `root` if you're mounting the backbone app on nested url. 

http://backbonejs.org/#History-start

you use this by starting a rendr sever with 
```coffee
app.use rendr.createServer(appData: {rootPath: '/cool-rendr-app'})
```

`Root` is a very generic name so I called it `rootPath`.