Added support for asset deselection delegate messages.

* Added [ELCAsset assetDeselected:].
* Added [ELCAsset shouldDeselectAsset:].
* Added [ELCAssetSelectionDelegate shouldDeselectAsset:previousCount:].
* Implemented [ELCAlbumPickerController shouldDeselectAsset:previousCount:]
  to forward to parent.


Allows consumer to determine if an image can be deselected or not.  Offers the caller the ability to know when an image was deselected.

In my case, I want to allow for a minimum of two images to be selected.  I'm disabling the "Done" button upon image picker view presentation, and re-enabling it when at least two images have been chosen.  In order to have enough control over this process, I need to know when an image is deselected.