Add CommonJS/AMD module definition

I needed to use the router from within a browserify project so I've created this fork which exports the router as a UMD module.  Not sure if there was a reason for modifying the default Backbone.Router rather than extending it, but in the context of a UMD module I think it makes more sense to return a different class.  If a module system is not being used then the router is now available as Backbone.FilteredRouter.