Do not trigger router/history events if Router#execute returns false

This might be useful for implementation of before/after filters. E.g. when filter is not passed it explicitly returns false to Router#execute and none of router/history events will be fired in this case:

```js

Backbone.Router.prototype.execute = function(callback, args) {
  if (!passedBeforeFilters(args)) return false; // No events triggered
  if (callback) callback.apply(this, args);
};
```