Enable possibility to pass an empty prefix

This feature could be used to create a main router that delegates unmatched paths to a "global" router. The main router would then only contains sub router definitions. I put an example usage at the bottom.

Tell me if it's something you would accept. Maybe there is already a better idea to achieve this. Sorry in advance for any missing things, first time I contribute to a JavaScript project :)

Example Usage:

```javascript
var Router = Backbone.Router.extend({
    routes: {
      // Sub paths
      "projects/*subroute" : "routeProjects",

      // Fallback to MainRouter
      "*subroute" : "routeMain",
    },

    routeMain : function (subroute) {
        if (!App.Routers.Main) {
          App.Routers.Main = new MainRouter("");
        }
    },

    routeProjects : function (subroute) {
        if (!App.Routers.Projects) {
          App.Routers.Projects = new ProjectsRouter("projects");
        }
    }
  });
```

Regards,
Matt