Routes.set_prefix()

Hi

I needed `Routes.set_prefix()` to `defaults['prefix']` be setable based on currently signed in user UID so here's little fix - hoping it'll be usefull for you too.. ;) 

So with this patch we can do the following:
```javascript
Routes.set_prefix('/ada46feca');

Routes.root_path()
>> '/ada46feca/'
```
js-routes doesn't depend on jQuery. You need to implement this method on your own.