Fulfil optional jQuery dependency from Backbone.$ instead of the global $ object.

Fulfil optional jQuery dependency from Backbone.$ instead of the global $ object.

If jQuery is used with Backbonejs, then it should be available as `Backbone.$`. Since we're already using `require()` to get Backbone, but do not want to create a hard dependency on jQuery, it's better to refer to `Backbone.$` instead of relying on a global `$` object.