The Model can accept the scope to do the url scoping, especially useful for nested resources.

The scope can either be a function or string. The generated url will be "scope/resources".

For example,

```javascript
Ticket.scope = "/projects/1"
Ticket.url() ->  "/projects/1/tickets"
new Ticket({id: 1}).url() -> "/projects/1/tickets/1"

Ticket.prototype.scope = function() { return "/projects/" + this.project_id }
new Ticket({id: 1, project_id: 2}).url() -> "/projects/2/tickets/1"
```