Added the listen function.

Added the listen function to Store interface so that people can use
Reflux the non-mixin way.

reflux/reflux.d.ts

@@ -35,6 +35,7 @@ declare module  RefluxCore {
         stopListeningToAll(): void,
         fetchInitialState(listenable: Listenable, defaultCallback: Function): void,
         trigger(state: any):void;
+        listen(callback: Function, bindContext: any) : Function;
     }
 
     interface ActionsDefinition {