Add start and end events for collapse and expand transitions.

[NateRedding]

There are plenty of use cases where one would need to know when a collapse or accordion section is finished expanding/collapsing. This PR adds some simple start and end events that are emitted around the expand/collapse transistions. This will allow users of the collapse or accordion widgets to know when the animation is done so that they can take further action.

[NateRedding]

@chrisirhc I see you added this to the 'Purgatory' milestone. Does that mean it is in limbo and is pretty much being ignored? Any idea when it might get looked at? It's a pretty simple fix, so it seems like it should be easy to get approved and merged.

[karianna]

Hopefully @chrisirhc will chime in shortly - my additional thoughts on this - Is there some documentation that would also need to be updated? Can that be added to this PR?

[chrisirhc]

I put this as Purgatory as I don't see it resolving any existing issues (hence, it's not a priority) and because this introduces a possible breaking API change in the future.

The current plan is to convert the collapse module to use the $animate service which would have a $animate:close event. This is to make use of the services provided by Angular to keep it consistent with its animation APIs.

I was working on #1444 before, but now it seems that the $animate service has changed since and there might be a more concise way of using the $animate service.

[wesleycho]

Since we migrated to ngAnimate for this, I am closing this as superceded - as @chrisirhc mentioned, $animate has events you can hook into if necessary.