Update backbone.d.ts

Added Backbone.History.started 

https://github.com/documentcloud/backbone/pull/538#issuecomment-15996793

backbone/backbone.d.ts

@@ -198,6 +198,7 @@ declare module Backbone {
         pushSate();
         getFragment(fragment?: string, forcePushState?: bool): string;
         getHash(window?: Window): string;
+        started: bool;
     }
 
     export interface ViewOptions {