MRH/SB 81377330 photoplus dependencies

https://www.pivotaltracker.com/story/show/81377330

Prior to this change, the code depended on a mixin from AG to get listing media data.
This version sends an event "uiWantsListingMedia" with a listingId when it needs media.
It listens for an event "dataListingMediaReady" with the media data.

After this PR is merged, there will be a PR on AG to integrate with this new version.
Coffeescript has a handy shortcut to name keys –

```coffee
@trigger 'uiWantsListingMedia', { @listingId }
```