Fixed issue with download operation preventing proper cancellation

When NSOperationQueue's cancelAllOperations method is invoked it calls "cancel" on all NSOperations it has queued. The problem was our custom FetchImageOperation wasn't overriding "cancel" so it didn't have a chance to clean house.
Fixed eternal spinner bug on details page
Prioritized details page thumbnail download - much quicker

Epecially the details page is brought up after much scrolling around the My Uploads page before all image downloads have finished. Also prioritized the full-screen image.
Merge pull request #80 from montehurd/offscreenDownloads

Prioritized details page thumbnail download