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.
As a side note - I see in the Mozilla.Pager docs that we also pass a native object to the constructor. This works in the case of that particular plugin, since it turns it into a jQuery object anyway. However I wonder if we should standardize in the examples we show in the docs? I think seeing as both plugins have jQuery as a hard dependency, we should probably just pass either a string selector or a jQuery object in the examples. Thoughts?