Fixed: CPImageView no longer leaks due to image load notification.

Previously, CPImageView would sometimes attach a notification to CPImageDidLoadNotification when the image was set, but would never remove this notification. The view could then leak memory as the notification center would be keeping a reference even if the view was no longer being used.

This commit fixes that issue by removing the observation once the image has loaded.

Fixes #2198.
Merge pull request #2218 from RoosterDragon/ImageView-leak

Fixed: CPImageView no longer leaks due to image load notification.