Improve mozilla-accordion.js. Bug 1055830.

- Add collapse/expand callbacks.
- Add ability to programmatically create/destroy accordions.
- Add mozilla-accordion.js tests.
- Add standard GA callback JS for existing accordion pages.
This should be passed a jQuery object instead of using a native selector? 

Perhaps we could make it so you can pass either a jQuery object, or a selector which the plugin can then turn into a jQuery object?