Js revisions to Accordion File

Hey @trevanhetzel, reworked the accordion file to do a few things:

1. Expose the Accordion object on a `Barekit` namespace
2. Expose a defaults that can be changed globally
3. Updated the jQuery plugin to not break chaining and attach a reference to the Accordion object to the jQuery data store.
4. Namespace the event so it could be cleaned up (in the `destroy` method)
5. Fixed an issue where selectors weren't scoped to the current accordion

There were other small tweaks and changes, but let me know if you have any questions. I am hoping this is enough to provide a pattern for the other JS files.

_As I mentioned on our call, the `core.js` file has been added to the gruntfile, but not gulpfile._